One of the best ways to measure the inflation rate is to watch the paychecks of the nation's 680,000 union-label postal workers. They have enjoyed bigger--and more frequent--raises than their white-collar government colleagues, thanks to a contract that guarantees them four different kinds of salary increases or payments every year.

But next month the U.S. Postal Service's clerks, letter carriers and other craft employes will need a magnifying glass to spot their cost-of-living-allowances, which will amount to 1 cent an hour.

Most of the Postal Service employes (about eight of every 10) belong to unions that bargain with the giant mail-moving corporation. Unlike white-collar civil servants, who must depend on Congress and the White House for annual pay raises, postal employes are guaranteed in writing an annual pay raise of $300 (each July), a once-a-year lump-sum payment of $350, plus COLA adjustments effective each May and November.

In addition to guaranteed raises and special payments, the Postal Service gave each employe a $150 ratification "bonus" (postal officials don't like that word) when union members voted to approve the current three-year contract that expires in July of next year.

The combination of pay raises, special payments and COLAs has made postal employes the envy of their less-unionized colleagues in other federal agencies. But the days of the fat COLAs may be over--at least for a while.

Last November, postals got a $541 annual COLA and $166 in May of last year, in addition to regular raises and special payments.

In November 1981 the postal COLA was $395 and in May of that year it was $686.

The May 1980 COLA was $832, and in November 1979 it was $749.

Postal union officials, along with keepers of the Postal Service's budget, have been watching the declining inflation rate as closely as rank-and-file employes, who see their raises dwindling.

During the long years of high inflation, Postal Service officials had worked up a plan to put some kind of limit on inflation adjustments (which had outstripped guaranteed annual pay raises) in future contracts. Postal union leaders took the view that their members liked the taste of the full COLAs and that would have to be continued in any future agreement.

Now that inflation has devalued COLAs, however, union and management may switch roles. If inflation stays down, the Postal Service could insist that COLAs remain a part of any new contract, while union leaders may decide to get bigger guaranteed raises rather than uncertain COLAs.