I once read a book about sociopaths and it said that the top sign that you're dealing with a sociopath is that you consistently leave discussions with them feeling confused and wondering if you're crazy.
The reason for that is simple: part of how we try to comprehend other people is by modeling them in our head. For earnest people with straightforward motivations that are similar to our own motivations, that modeling is easy and we feel comfortable satisfaction each time our mental model helps us predict their behavior. But for people who are deranged and have ulterior motivations, developing a model is difficult (or even impossible for those of us incapable of significant malace or deceit) and we become profoundly uncomfortable when we are unable to make predictions about their behavior.
Of course, its possible to sometimes leave conversations with good people feeling confusion and self doubt, and also a lack of those feelings doesn't prove someone isn't a sociopath, but I find it useful to privately contemplate whether someone might be a sociopath if they consistently make me feel like I might be crazy.