When Ronald Reagan first announced his proposed budget cuts more than a month ago, he pledged full protection of the poor.

"Those who through no fault of their own must depend on the rest of us, the poverty-stricken, the disabled, the elderly, all those with true need," Reagan said, "can rest assured that the social safety net of programs they depend on are exempt from any cuts."

The message won strong support in many quarters, but for critics there was a crucial question: Who are the truly needy?

For people like Bill and Marilyn King, Pauline Herndon or Rose Ricks, the answer seems to be clear: They will not be among the new and protected class of poor. In some way, each family will suffer next year if Congress approves Reagan's proposed cutbacks.

Pauline Herndon would be the least affected since she receives aid from only one federal program, food stamps. Starting in January 1982, if Reagan's proposals are approved, she would lose $14 in food stamp benefits. If she worked 40 hours a week, our four more than her regular work week of 36 hours, she would lose all her food stamps.

"I don't expect things to get better," Herndon says wearily, as she turns to hug her son Ricky, 9. "I'm only trying to make sure my children will survive."

Rose Ricks would come under Reagan's proposed "workfare." That plan, announced recently, would require most welfare recipients, including single parents who recieve Aid to Families with Dependent Children (AFDC), to work for their benefits. In Ricks' case, she would have to work at least 20 hours a week at a government-chosen job to keep the $344 a month she receives from AFDC.

The "workfare" plan probably would force Ricks to attend college only half-time, instead of full-time, thus reducing her student grant to at least half the $760 a year she now receives.

Ricks also could lose at least $24 a month in food stamp benefits because her children receive free school lunches, and changes in federal formuals for subsized housing would increase Rick's share of her rent by 5 percent. Currently, under the federal Section 8 program, she pays $229 of the $339 monthly rent, while the rest is paid by the federal government.

Local officials estimate nearly 3,100 Northern Virginia families benefited from federal housing programs last year.

"All I want to do is get ahead and get out of this hand-to-mouth life style," Ricks says, "but first I need the knowledge. It's going to be hard under Reagan, that I know, but 'm going to make it. I have to make it. I've seen such a change in my daughter since I've returned to school. She's really proud of me and has turned into a beautiful little girl I have to make it for her.

"What scares me, though, is what the cutbacks are going to do to the black community. Already I see so much depression and frustration. These kids look around at their parents who are really trying to do something, but never seem to get ahead. They can take on the attitude, 'Why should I try? They've really tried and look where they are -- still on the bottom.'"

Marilyn King and her granddaughter Tiffany would be hardest hit by Medicaid cutbacks. Marilyn King requires daily insulin injections; she also requires medication and frequent visits to the doctor because of her asthma and cerebral palsy. Most of the costs are covered by Medicaid.

Tiffany must be fitted for an artificial limb, and doctors say she is likely to need several operations and prolonged rehabilitation. Most of those costs, too, would be borne by Medicaid.

Under Reagan proposals, Medicaid payments would be reduced $100 million starting in September, allowed to increase 5 percent in 1982 and only at the rate of inflation in following years.

State and local officials say those cutbacks could be among the most devastating for poor. In the last decade, as more people qualified for Medicaid and as medical costs skyrocketed, the program in Virginia has increased tenfold -- from $27.4 million in 1970 to $287.3 million in 1980. Last year, nearly 11,000 Northern Virginians received more than $13.4 million in Medicaid benefits.

Critics say the proposed cutbacks overlook two crucial points: Continually rising medical costs and the fact that no one will be cut from the rolls. In fact, they say, since federal eligibility guidelines will not change, more people can be expected to qualify. The net result, according to critics, is that states will have more and more people with higher medical bills to serve, but drastically fewer funds to cover their needs.

"I don't understand why Reagan is doing it. He speaks a language I don't understand," says Marilyn King. "If we were rich or if were able, I wouldn't take a penny of his money. But we aren't.

"I must go by faith and I'm hoping and praying that none of this will happen. Or that God can foresee something good from this that I can't see. People aren't going to starve to death."

The problems of the Herndons, the Kings and the Rickses are not unfamiliar to state and local officials trying to determine the scope of the proposed cutbacks. But they have few answers when they are asked to pinpoint exactly who will be protected by Reagan's "safety net" of social programs.

"As far as we're concerned, everybody who is now receiving some type of federal assistance is needy. But where the cuts may be and how they're going to hurt someone is an answer we don't know," said Robert Osborne spokesman for the state Social Services Department. "We just have to wait and see."

Critics of the cutbacks are more precise.

For example, the Project on Food Assistance and Poverty has estimated that more than half the 25 million Americans who fall beneath the adminstration's official poverty line -- $8,400 yearly income for a family of four beginning July 1 -- would not be protected by the "safety net," or at best, would receive a free school lunch. The Project is a nonprofit group formed recently to study the problems of hunger in America.

"What's extraordinary about the Reagan proposals is that it is precisely the neediest people who are hardist hit . . . and for many of these people, they're not being hit just once. They're being hit again and again," said Robert Greenstein, director of the food stamp program under President Carter and now director of the Project.

The Kings, for example, participate in five federal programs, including four where cutbacks have been proposed. The Ricks qualify for seven programs; Reagan has proposed reductions in six of those.

Reagan officials concede that there is no magic formula to determine who is truly needy. Some administration officials estimate that one-sixth of all single parents receiving AFDC will no longer qualify for aid or will have that aid reduced. In Northern Virginia, more than 13,000 people now qualify for AFDC.An estimated 2,100 people would be cut.

About 40 percent of all people receiving food stamps will be cut from the rolls, according to administration estimates, while benefits for the remaining 60 percent will be reduced. In Northern Virginia, more than 25,000 people receive food stamps and an estimated 10,000 people would be cut.

Still, the administration says, those who truly need help can depend on protection from the "safety net." This is defined by Reagan officials as including seven programs: Supplemental Security Income, veterans compensation and benefits, Medicare for the elderly, Head Start, school lunches, Social Security and job training for youths.

But even the "safety net" has not escaped the eye of budget cutters; three of the programs -- school lunches, Social Security and youth job training -- were earmarked for cuts by Reagan. (Last week, the Senate Budget Committee refused to endorse all the Social Security or job training cuts, but agreed to the president's school lunch reductions. The full Senate was to begin debate on the proposals this week.)

Some critics believe the central problem with the administration's proposals, safety net or not, is trying to design programs only for the "truly needy."

"The reason no one can predict who the truly needy are," says Ken Bowler, as assistant to the Democratic chairman of the House Public Assistance committee, "is that with the exception of the aged, blind and the disabled on SSI, nearly everyone receiving some type of federal assistance will be hurt."