While it is possible to pop an adult BP it should only be attempted by seasoned professionals... think of hatchling snakes as rubberbands, it's harder to hurt them and they tend to bounce back. Older snakes, and bp in particular because they are so heavy bodied, can be hurt more easily because they are stiffer and no longer growing/healing at a rapid rate.
Bottom line... cough up a few bucks and take them to the vet... If you can't afford a simple gender check then you have no business breeding anyway, and if you're not breeding then why do you need to know the sex.
Just my humble opinion.