It is not easy to determine the sex of a snake. This three piece set of probes can be used to work out if your snake is male or female. This is important when placing two snakes together.
The probe is inserted into the vent (sometimes called the cloaca). If it won't go in very far then the snake is probably female, if it will slide in then it's probably male as the male sex organs leave a space in this area.
This high quality set contains three probes in a useful case. Well made and durable this kit is available at a great price from Swell Reptiles.
If you are at all unsure of how to sex a snake, then it is vital to check with your vet. The procedure can cause injury if not done correctly.