www.sassnet.com has links to the current rules handbooks (shooters and range officers handbooks).
Gas checks are not allowed, for the reason you supposed (except in some long range rifle side match events). As far as telling if a loaded round has a gas check, I guess if a shooter made a mistake, folks wouldn't know until the shooting started. Although the consequences are listed under a different section, I believe they apply here as well:
"Shooters may be held responsible for damage caused to a target or injury to personnel due to "bounce back" because of inappropriate ammunition. This major safety violation is grounds for "instant disqualification" and ejection from the match."
Specific to main match revolver and rifle ammo bullet selection, the shooters handbook states, "Revolver and rifle ammunition may not be jacketed, semi-jacketed, plated, gas checked, or copper washed. It must be all lead. Molydisulfide coated bullets or equivalent are acceptable."
Get in touch with a local club (also listed at sassnet.com), and see if they can set you up for an orientation class, or at least watch some matches for fun. They may also be able to suggest an experienced CAS shooter that you could turn to with any questions not covered in the handbooks. Some of the conversations held on the sass wire forum at sassnet.com don't really reflect the attitudes found when you attend matches, but you may also glean some useful information there. If you ever decide to join SASS, they'll send you a copy of the most recent shooters handbook, too.
I haven't ever seen anyone use the wrong type of ammo at a match. Most bounce back comes off of shotgun targets, which is why pellet size is restricted to smaller sizes.
