When President Reagan flew into Fort Wayne, Ind., last week to lift sandbags and lend moral support to the victims of extensive flooding there, a team of investigators from a federal agency was already in place, preparing to answer a question that was perhaps uppermost in the victims' minds.

The question was answered Saturday when the president, on the recommendation of the Federal Emergency Management Agency, declared Fort Wayne and surrounding Allen County a major disaster area.

That designation can mean millions of dollars in grants and loans, free legal assistance, housing, clothing and medical services to a state or locality.

But the designation doesn't come easily. FEMA says it only approves about half the requests it receives. Earlier this year, California, Maryland, Missouri, Alabama and Washington state sought help because of damage from rains, snow and floods, but FEMA qualified only California.

A major disaster declaration is supposed to be made only when state and local governments are incapable of coping with the disaster on their own. The law defines such situations as natural disasters, fires, explosions or "other catastrophes." Under the latter provision, FEMA provided disaster aid to residents near the Love Canal and to Florida to help cope with the influx of Cuban refugees--actions that prompted some congressional criticism.

"In California it was quickly evident that we had to mobilize to save lives," said Lee M. Thomas, associate director of state and local programs and support at FEMA. "There is a difference between houses being washed away versus the need for legal assistance."

A process that leads to winners and losers inevitably has its critics. In December, the General Accounting Office suggested that FEMA should use computer models to ensure that its evaluations are consistent. FEMA says that idea is impractical because every disaster is different. But it does plan to adopt a GAO suggestion that its procedure be standardized.

The administration has proposed $325 million for the agency for fiscal 1983--about the same amount as fiscal 1982--but the agency's funds are administered differently than most agencies'. FEMA has a revolving account, which carries over unspent funds from previous years. The agency can also seek additional funds from Congress whenever its revolving fund dips close to $200 million, as it did after Mount St. Helens erupted in 1980.

To earn the disaster designation for their states, governors have to provide FEMA with an assessment of the damage and their ability to respond. The assessment is supposed to include the number of casualties, number of homes damaged, the number and type of businesses and farms affected and the extent of damage to public buildings, roads and other structures.

The FEMA team (usually about six people for a disaster the size of Fort Wayne's) arrived in the city after the state made an "informal" request for aid. It was too early for the formal request--"it's still raining, the snow is melting and the water is still up and has not crested yet," FEMA spokesman Bob Blair explained last week. "When a road is covered by water, you can't tell much about the damage. It may be destroyed or it may not be touched."

Once a disaster is declared, FEMA is responsible for coordinating federal and volunteer agencies ranging from the Internal Revenue Service to the Salvation Army.

It provides aid to state and local governments to help repair and replace facilities like roads, bridges and buildings, as well as assistance to displaced families.

Individual assistance is tailored to the disaster. For instance, if there are homeless families, FEMA tries to put them up in hotels or vacant apartments. Families can sometimes can get rent-free housing for up to a year.

If there is no available housing FEMA can bring in some of the 3,000 mobile homes it has scattered around the country. But the agency uses them as a last resort because of the time and money it takes to find a site for them, level the land, obtain the proper zoning and provide utilities and roads.

If families are left without furniture, the Red Cross or Salvation Army will often supply basic items. FEMA can also arrange for the federal government to buy some, and loans from the Small Business Administration and Farmers Home Administration can also be arranged.

Homeowners with more limited damage can qualify for funds to make their houses livable until loans or insurance money come through. SBA loans can be obtained for home repairs at an 8 percent interest rate for those who can't obtain funds elsewhere and at 16 percent for those who can. In rural areas, FmHA loans are available to pay for repairs to dwellings and machinery and to replace livestock.

Under a program known as the individual and family grants, victims can get funds for things like eyeglasses, dentures, medical bills or funeral expenses. And during a food shortage, the Agriculture Department can issue emergency food stamps or distribute food directly.

Disaster unemployment assistance is normally available to those left jobless who do not qualify for state unemployment compensation, such as a self-employed person like a store owner.

He can also apply for an SBA loan of up to $500,000 to restore his shop. The interest rate is 8 percent for businesses that can't get help elsewhere, and 15.75 percent for everyone else.

Volunteer lawyers are provided by the American Bar Association to give free advice to victims at assistance centers that FEMA sets up in disaster areas.

And the IRS normally provides staffers at the assistance centers to help victims amend their tax returns so that most can qualify for a tax refund because of casualty loss deductions. The IRS tries to get the money in the victims' hands in three to four weeks.

The IRS services seem to surprise people the most, according to Blair. "People usually look at us like we're crazy when they come into the center and we refer them to the IRS section. They say, 'The Internal Revenue Service! That's the last thing I need.' "