I believe that the checkers can tell if one is even eligible for Pre-Check on a given airline as well as if they qualify for this particular flight. For people who are not even eligible in a given case, and try to scam it, the best approach would be to send them out of the line. For those that are eligible, but don't qualify on a given flight (no general way to know without scanning barcodes), jumping into the line is reasonable.
Cheers.
That is not true. In many airports, the checkers have no machines, they are just guarding the line to only accept elites (assuming the only Pre fliers must be elite). It's very easy to just say to those checkers one is GE, and gain entry.
Even if the checker has a scanner, one just gets a CLR if one is not in the program, so one can gain a shorter line but be thrown into the non-LLL machine/process.