Ahh...good question. First of all...interestingly, garters urinate a kind of solid looking powdery, milky white stuff called urates...so if you see that come out of her tail end...that is not poop. If you actually see her doing her business, you will see her lift up her tail and deposit either urates or brown/black stuff..which is poop...or it could look exactly like whatever she ate...we actually had a thread about it: here you go...poop pics:
Poop ID.