Are you trying to determine gender of live beaver or dead beaver? If on live beaver, I have about a 50% chance of guessing correctly. On a dead beaver, the sex can usually be determined by pinching the skin just above the castor gland.
If you are not saving the castors, open the skin to expose the castor gland. If it is a male, you can see its manhood. This is shown in picture 5 in the below link. This is what can be felt by pinching the skin, above the castor gland, on an unskinned beaver.
http://www.trapperman.com/trapperman/beaver_castor_removal.html