Ditto on the palp method Mike mentioned. My BRB pair were sexed by two different very reliable breeders when I got them as babies and so they worked well for me learning what to feel for in the males. Once you notice what the bumps feel like they are hard to miss.
Also I would never suggest bringing any animal to a show unless you are either a vendor there or returning an animal to a vendor. Every show I have helped with or attended had strict no outside animal rules. This forces whoever you wanted to deal with the animal, assuming they are willing, to come outside the show to help you. Thats on top of environmental stress to the animal(although if your carefull that would be minimal).
The one sexing option I would suggest as an alternative to vets of unknown skill is to try and contact a reliable breeder in your general area. Even better if the breed rainbow boas. In addition to word of mouth you can possibly locate such a breeder by talking to the employee's/owners of any local pet stores which seem to be knowledgeable and skilled with herp's. If they actually are good with the herp's they sell then they should know both reliable local breeders and skillfull rep vet's that could be trusted to sex without damaging the animals.
Good luck,
Sean.