Unless the person has voice comm, stay away from them, if they do, just listen to their voice, you will easily be able to gauge if a person is lying, or a threat, or friendly, by their tone of voice. Humans are very intuitive naturally, you don't even have to think about it, you can pretty much feel if something is "wrong" and if you get that gut feeling, get the hell out of there or put a bullet through his head. If you're undecided, just keep him talking, you will pretty quickly get a "feel" for a person, especially when you don't have physical appearance to distract you.