how can I tell what volt the alternator is?

Hello, showing one belt for a 1994 Suburban 1500 with a 5.7L V8 engine regardless of alternator application. However, I am thinking you are saying 120 amp, not volt. I show a 100 or 110 amp alternator available for that year from GM. If this is not an original equipment alternator, I cannot say for sure what the pulley would look like, but my guess is that the belt would be the same regardless if the part was engineered for the vehicle application.

With or without air pump..