I can tell by the eyes and body language. The eyes are very important. Are they constantly looking back into your eyes or are their eyes wandering around. Are they smiling with their eyes or do they look empty? Body lanugage is also a good one. Do they always stand close to you or are they a step back, do they smile at you alot, or lean toward you when they are takling to you. Their are all kinds of different things that you can pick up on when you look for signs.