How do you tell the gender of a 3 moths old stimson?

There are three ways; probing, popping, and "feeling" the snake. Probing is when a lubricated probe is inserted into the cloaca, the length of which the probe enters until it meets resistance will determine the sex. Popping is when the hemipenes of a male are "rolled" out, if nothing comes out it's a female. The last one is when you stretch out the tail after the anus and put firm but gentle pressure on it's tail, then move your finger along the tail away from the anus. If you feel something like a rubber band snapping then it's a male. This technique only works with thin-tailed species. The first tail I would only suggest if you have experience doing them.