People put out signals--body language, gait, clothes, facial expression, posture, attitude, speech, mannerisms--that can tell you where they're from, what they do, who they are. Most importantly, do they fit in. Because if you don't fit in, the target will spot you, and after that you won't be able to get close enough to do it right. Or a cop will spot you, and you'll have some explaining to do. Or a countersurveillance team will spot you, and then--congratulations!--the target will be you.