The quickest way to tell whether someone is worth listening to is if they believe in unsubstantiated conspiracy theories. This applies to many far-left people, too, though leftist conspiracy theories tend to be within the realm of plausibility even if they aren't really backed up ("wealthy people are doing [X] to keep the worker down"; such claims necessitate evidence but they're not inherently outlandish and bizarre on their face), while rightist conspiracy theories are near-universally batshit insane fever dreams with no connection to reality.

There's high correlation between profound mental illness (psychotic conditions) and belief in rightist conspiracy theories. You can also see this in the fact that they almost never believe one or two or three: someone either believes in dozens of these absurd things or none at all.

An easy tell that you're dealing with one of these people is if they jump on you for using the term "conspiracy theory". This term triggers them and incites an immediate reaction. They will come out of the woodwork as soon as you say it. Seeing/hearing it makes them feel a flash of impotence and rage, which they feel a strong need to repel so they can return to a more positive and self-assured psychological state. Even if it wasn't directed at them or at anyone, simply encountering it at all is enough to generate these feelings along with a reactive self-soothing response.

(Interestingly, some also react similarly to someone just saying "chud". A proportion of them get oddly defensive and emotional.)

It also works in reverse: if you encounter a right-winger who doesn't really believe in any conspiracy theories, they might actually be the rare intelligent right-winger and are potentially worth hearing out and/or being very wary of, because they're more capable of being effective than the rest of their bumbling oafs of allies. Curtis Yarvin and Peter Thiel are two examples of this. There also may be a psychosis-psychopathy trade-off, here.