Usually the first thing I notice is their energy. On ground level I need to know if this person could be a potential enemy or friend towards not only myself but to people I love as well.
Second I gauge how confident they are about themselves and their thoughts by using various techniques (how they shift their eyes, how they blink, how they walk, posture, talk, how they react and the list goes on).
To tell you the truth I've never met anyone that has kept the enemy vibe, if I see it coming I'll neutralize it and shift it towards friendship.
Even beings such as devils would be powerless to the embrace of true love.