Trek 6500 rear derailleur
If you are looking for an exact replacement the part # will be stamped on the inside (wheel side) of the parallelogram linkage. This is the middle section of the derailleur it is the part between the mounting or "B" knuckle and the jockey wheel cage. The numbers will be small and are read more easily with the derailleur off the bike. It should begin with RD followed by four digits or RD M followed by three digits. You will also want to measure the distance between the jockey wheel mounting bolts to get the correct cage length as this will effect tooth/gear capacity.