Just look through it- like sift through it with your hands and see if you see any tiny black smears. Changing it every few weeks is plenty.
My snake that I rarely find poo from is around 50 grams, I think. Same thing with my Eastern Hoggy- just barely over 50g, and I've just started finding tiny poos.
Now when the 460 and 760 gram snakes go- you know the minute you walk into the room!!
Nanci