I'm an avid people watcher. I catch other people watching me too, and I have learned that it can be done in a pleasant appraising manner without invading the other person's privacy and causing them to get all creeped out. I've been on the receiving end of that sort of behavior, usually it's a male and I can almost read their mind, as if they are thinking [WTF is that, is it male or a female?] I'm rather good at deflecting their gaze when that happens by saying, [WTF you looking at?] They invariably get embarrassed and scurry away.
People watching can be very instructive, there is a lot to be learned by watching body language and styles of dress. Starbucks is a prime place for this, I try to avoid eye contact because it's the initial event which oftentimes is followed by a smile which on occasions has led to a conversation and even an unwanted relationship. Since I'm married I owe it to the S.O. not to allow it to go there. Just the same if you want to learn more about people, gender dynamics and dress modes that you might wish to emulate, and hour of people watching a day is a good exercise.