As Paul Simon might have written, there are 50 ways to tell that the recession has started.

At the Business & Finance section of The Washington Post, there is usually a substantial and sustained increase in telephone and mail volume as the economy turns sour. The mail arrives in several batches a day now, each time in large boxes. And, as too many readers know, there is a telephone overload.

But, in contrast with earlier economic declines, the urgency in the voices of callers today is more pronounced. The press releases from businesses are longer and approaches by their spokesmen seem more frantic. Every publishing company in America appears to be coming out with a new book on hard times -- a business that must be good, since so many authors are traveling all over the country to promote them.

A seat-of-the-pants judgment, and only that, would have to be that for the first time since early years of the Depression, metropolitan Washington's economy is weakening in concert with that of the nation as a whole. No longer is the D.C. area an isolated land of milk and honey, to cite a description used by the Sindlinger organization in the 1970s.

Because there is no regularly updated, valid base of economic data about metropolitan Washington, the statistical confirmation of weak business and income trends here probably will lag many months behind nationwide figures which show that a recession is under way.

However, there are lots of little bits and pieces of information that add up to a very troublesome winter and spring for Washington business.

Among the most fundamental figures are employment and personal income. On both counts, the boom economy of Washington has ended. The number of jobs has been shrinking, mostly in the District. Overall unemployment is up about 15,000 jobs in the area since last year but a close reading of all the recent employment data here seems to indicate that some people are simply moving away and the labor force is shrinking, too. Thus the percentage of unemployed persons remains low by national standards (about 5 percent vs. 8 percent).

The most recent personal income statistics, released by the Commerce Department yesterday, also show weakness. Nationwide in the second quarter this year, total personal income (all wages, salaries and other income to individuals) rose 2.1 percent after a 2.9 percent rise in the first quarter. In the District, the second-quarter gain was 1.5 percent; the gains in Maryland and Virginia were 1.8 percent and 1.4 percent, respectively. For the past year as a whole, only Virginia matched average national personal income growth figures with Maryland and D.C. at lower growth levels.

There is more:

* Area store owners and managers, restaurant operators and hotel executives say business has been weak since August but that, in October, consumer spending seemed to stop. Parking lots at such huge shopping centers as Montgomery Mall have not been crowded on weekends although the Georgetown area in the District appears to be an exception in terms of shopping crowds -- a tribute to the new Georgetown Park mall there.

Hotel occupancy rates are down in the District and suburban Maryland for the first eight months of 1981 although there was a slight gain at Northern Virginia hotels. In August alone, hotel occupancy was down more than 6 percent in the area compared with 1980.

* Sales of new and old houses are very slow because of high mortgage interest rates. In August, construction was started here on 1,280 new housing units (1,110 of them single-family houses) with a total value of $56 million; a year earlier, not a banner era for housing, 2,336 housing units were started with a value of $80 million.

In the commercial real estate sector, there has been a very significant increase in office and industrial space vacancy rates -- at a time of booming construction activity that could portend bigger problems next year as more new offices are ready for occupancy.

According to the real estate firm of Coldwell Banker, area office vacancies in September were at the highest level since mid-1978. The vacancy rate of 0.8 percent indeed is slim compared with the nationwide vacancy rate of 4.4 percent and the 7.9 percent rate of such centers as Kansas City and Phoenix. But the 0.8 percent rate is four times the 0.2 percent vacancy rate of last June and the vacancy rate here hasn't been higher than 0.3 percent since March of 1979.

The industrial space vacancy rate here also is low by U.S. averages, but the 2.3 percent level in September was the highest since March 1978 and up from 2 percent in June.

* One of the most negative local economic indicators is the index of help-wanted classified advertising in newspapers, compiled monthly by the Conference Board in New York and normally a very sensitive and accurate barometer. The Washington-area index plummeted in September to 103 (the index starts with 1967 as a base of 100) from 117 in August and 134 a year earlier. A similar sharp decline took place throughout the Southeastern United States but the nationwide decline was not so sharp.

No wonder that a Washington businessman, in quiet conversation last week, was extremely pessimistic about the economy.

As it turned out, he correctly predicted the outcome of Virginia's gubernatorial election as Democrat Charles S. Robb won handily.

But it was the reasoning behind that forecast that helped to explain the depth of fears and concern in the business community about where current federal policies are leading us.

As this business leader sees it, the Democratic victory in the Commonwealth last week will be seen as the beginning of a new direction for the right-left economic pendulum. Less than a year after President Reagan's inaugural, under this scenario, the pendulum's swing away from liberal spending and centralized, strong government has been stopped and is headed in the opposite direction.

What this means to some business leaders is that the administration's economic program won't be pursued as promised. With no change of policies, a very deep recession is considered possible and this region won't escape the pain as in earlier years. But if changes are ordered, there probably will be more federal spending and a revival of inflationary expectations.

This is bitter medicine to a business community that overwhelmingly supported Reagan and likes him to this day. Moreover, they have endorsed as enthusiastically as possible the economic policies the administration has followed, in public speeches and articles if not by corporate spending allocations.

It is only when some business people sit down for a private conversation that they express other thoughts -- primarily their belief that the reality of politics in Washington and in congressional districts across the country will bring about radical changes in Reaganomics, and soon.