I'm not sure about this, but please do not do any probing if you dont know even this! Probing can really hurt your snake if you dont know what you are doing.
Plus, if its a boa there are easier ways to sex them, ie. the spurs on a male boa can be felt with your fingers at the 'cloaca' or slit near the end of the tail.
Ok probably should have said what's the minimum age to get a snake probed? in the post but firstly totally agree with you on the first point i have kept snakes for 20 years but since have never undertaken any formal study of herpetology would never try such a thing i simply don't want to stress the animal by taking 'her' to the vet or other professional if she is simply too young secondly when sexing them by feeling for the spurs in a male then i believe that this is also difficult in small snakes case in question the breeder wasn't completely convinced 'she' was a she.