At the local NASCAR level they check with a durometer and a sniffer. If the hardness is out of spec for any reason you are DQed, suspended and fined. Inex, at least around here, does the same thing with the Legends, et al. In the regional and national touring series there are strict tire control rules and most of the time you don't get the chance to take the tires away from prying eyes for chemical prep. They also have facilities at the R&D center in Concord to compare chemical makeup. Karting and dirt track and "outlaw" circle track is where most of the tire cheating happens.
There are tire chemicals that pass the sniffer, unfortunately. But youre right, the sniffer is being used. Nascar has contacted WKA about their new sniffer. Which is very good actually. It can detect windex on your tire. For now, tire chemicals are none existent in WKA National events. You can almost throw away the durometer test, I can treat a set of tires and have the durometer be the same. Karting is horrible with the tire games. It has become one of those things where if you dont have the right stuff, you might as well go home.
The GCR says no modification of the tire. Only modification permitted is for a street radial to be shaved or "Trued"