Good questions, that are at the heart of this subject.
Scientists are people who rigorously adhere to the scientific method for validating and/or gaining provisional knowledge.
Anyone who doesn’t fit the definition above is not a scientist. Anyone who does, regardless of affiliation/accolade/credential, is.
As for who to believe/trust, the answer is none of them. Science is hindered by trust/belief (it’s called bias), and nothing should be accepted as correct without thorough validation first. The default position ought be one of doubt/skepticism. One of the things that makes science so wonderful is that it is independently verifiable/repeatable (or it is not science). The best (and sole) way to evaluate a scientific claim (regardless of source) is to evaluate and repeat the demonstration/experiment yourself - however ubiquitous scientific illiteracy (ie the acute lack of support), among other things, makes this challenging.
Deciding things by consensus, or “democracy” (“all the experts agree”, “science says”, “9 out of 10 dentists...”), is anathema to science and an incredibly stupid, dangerous, and unscientific thing to do.