The easiest way is check the vacuum line for trans fluid like crazy said. The thorough way to check, even if there is no puff in the exhaust, is remove the modulator [1 13mm bolt], and push the plunger in, if trans fluid comes out the vacuum line, it'* in need of replacing. If none comes out the vacuum line, then you will need to look elsewhere. But because you said when you come to a stop, that sounds like a typical cause of modulator failure. They are cheap, and the new one you buy is adjustable, so you can crank up some nice firm shifts for longer trans life.
'89 SSE - GX3 -190k, 155k on body - Back for another ride.