The best thing i can tell you is learn the class(es) of the fish you intend to compete in. There are little differences from class to class that the breif there kind of misses, specifically Traditional and asymmetrical plakat will not be "perfect mirror image" and DT bettas should have a much shorter body than any other class, ect.
But Vill is 100% correct, and Mo you're kind of hitting on why there is a "show quality betta" and ones that are not show quality. Those things Mo mentioned are an outright disqualification, along with the fact that any fish entered regardless of class must be at least 1.5". You can send whatever you want to the show, but if it's not at least pretty good form, or if it's got a feature that gets it disqualified, you may as well have sent it for nothing.
That being said don't be too wary about sending fish in. Lots of fish have faults, and if it doesn't they'll invent new ones to set them apart. Sometimes you may even find you're the only one entered in your class, or even see that no one is competing at all in another.