Boas and Pythons have spurs on either side of their vent males spurs are bigger than females,you can have them probed, but ALWAYS have it done by someone with a lot of experience, alternatively you can ask when you buy it,(sorry)
Most accurate is probing. It basically involves having some probes, lubricating the end with KY jelly or Vaseline or something similar, and sliding it down the entrance of the vent towards the tail. The depth it goes in determines what sex it is. i.e the further it goes in means it is male, whereas with a female, it would only go down a few scales. The depths are different for different species, e.g royals - males will probes around 6-8 scales deep, and female 2-4. Its pretty obvious once you know what you are looking for. But you should only do this if you feel confident enough to do it and have seen someone with experience do it.
Another method with hatchlings is popping. This means basically popping out the hemipenes on the male. You hold the snake and place your thumb just below the vent and roll your thumb up towards the vent hopefully everting the hemipenes. The tail is sometimes bent back slightly to help evert them. This should also only be done if you know what you are doing because it can cause damage to the snakes spine, and the hemipenes :lol:
Hope that helped, and if I got anything wrong Im sure some more experienced people will let you know :lol: :lol: