You used to be able to tell by looking at the 'front' of the engine (where the pulleys are) A chain ran in oil so there would be a metal sealed cover to keep the oil in, wheras all belts were dry so only had an unsealed plastic cover. But now there are rubber timing belts that run in oil (which is proving disastrous)
So if its an unsealed plastic cover its a (dry) belt, but if its a metal sealed cover it could be either.