The pan will be your biggest indicator. If it's a stock G-Body it will likely be a 350, 200, or 2004R depending on model & year.
1 Aluminum Powerglide 14 bolts
2 TH200 Metric 11 bolts
3 TH350 13 bolts
4 TH400 13 bolts
5 TH200-4R 16 bolts
6 TH700-R4, 4L60, 4L60E 16 bolts
7 4L80E...