That's why in many grassroots-style racing in the US there are "claimer" engines and protest rules. If you think someone is cheating, you can protest... if you are right, they pay for the inspection and the fine. If you are wrong, you pay for the inspection.
In a claimer engine class, you can pay a relatively nominal fee and buy an engine from say, one of the top five finishers.
Not that claimer engine rules really keep anyone honest... just honester. The story goes that at a small dirt oval with claimer rules, there was a guy with a big budget who won quite often. The noise against him grew enough that someone eventually claimed his engine at one of the races he didn't quite win... it was full of junkyard parts.... You pays your money and takes your chances.