Ask for a community meeting, so you can see that those people are real.
Despite that, I don't see any effective counter measure in the long run.
Currently, sure, with a keen eye you might be able to spot characteristics of one or the other LLM. But that'd be a lucky find.
I'm nitpicky about the word "believe". So let me rephrase: I do not believe. Either I know, or I don't know. Everything else are more or less informed speculations, assumptions or hypotheses at best.