If they just talk about the subject very cogently then I take them for an expert. No need to claim it. If they claim it and especially if they ask for your trust, yes, be suspicious.
Discussion
the problem is somebody could be regurgitating a valid talking point and appear intelligent but they're just parroting
to really show you're an expert you would have to have both quality AND quantity to your speech
that is you would have to say A LOT of things AND be on point in saying them
but time is too precious to waste it on blah blah blah