No one can ever be 100% sure, but body language does say a lot about what people are thinking, meaning if they are just talking to you and telling you what they think or whether they’re trying to bulls*t you. You can also find out a lot by what they actually say. But you have to engage people in conversation to find out how they are. You can’t just sit back and only let them talk (although you can figure out the really bad bullsh*tters that way, because they’re not subtle). It’s very difficult to tell how people online really are, because you don’t get to see their body language or hear the intonations and inflections in their voice. Plus words on a screen, can sometimes come across very differently than real words said in person.
I would have a hard time trusting someone who just started blurting out dramatic or personal things to me, especially if they were gesturing wildly. If I’ve just met someone, I need to take time to get to know them and for them to get to know me. I’m pretty honest and straightforward, but I don’t give out personal info to anyone, immediately. Even if it’s someone that seems OK.
I also never trust anyone who wants to know all sorts of personal information about me, right after we’ve met, or someone who gets irritated if I don’t tell them everything they want to know about me, in the beginning. That’s a huge red flag.
If a person is talking to you and seems to be talking non-stop about themselves, either complaining about how they’ve been wronged, or the opposite of that, telling you how fantastic they are, that’s a sign to you that that person is very self centered and probably doesn’t much care what you think or feel. That’s not to say that very chatty people, in general are like that. Two of my best friends could knock out an all night filibuster, but they’re very interesting women and they engage others in the conversation, they don’t become the conversation.
Be aware of overly dramatic statements of any kind. And be aware of too much flattery (towards you) too soon, especially if it’s rather personal or potentially sexual in nature (those are big red flags).
Also beware of people who make big bold dramatic statements (that sound mean or scary, or extremist, like political statements or racial statements or statements against one group or another) and then, when you react in a shocked manner, tell you that they were only kidding, or that you need to lighten up or that you can’t take a joke. Walk away from those people, quickly!