How can you tell what Amp alternator is in a 1996 Tahoe?

I don't know where to tell, but replacement alternators range from 100-140 amps for that model year.

On the top & to the right of the alternator you will find some numbers look for 3 numbers followed by an A which stands for Amps such as 105 A. The numbers are usually hard to see, you can take a piece of white chalk & rub over them to get them to show up better.