Between the contest rules and the Modeler's Guide to Competition (as Nick reminded us, it is the document formerly known as the Competition Handbook), most questions should be answered well in advance of any model show.
Rules. Huh! What are they good for? Well, absolutely everything...
They get updated at the National level annually (more or less), and most other IPMS-sanctioned contests base their rules on the Nationals rules, so you need to read them in advance of any contest. Ask questions. If the rules don't suit you--if you have some fundamental disagreement with them--you have the option not to play, or ask for some "Display Only" space. More and more shows offer it these days...you may still have to pay an entry fee, but you can still show your stuff.
The information is readily available. As Brad Hamilton told Jeff Spicoli in the movie "Fast Times at Ridgemont High", "Learn it. Know it. Live it."