Who is doing the looking? Whose shoulder? If someone is looking over their own shoulder, then it's the meaning Dave and Roi have described: watch out for danger, trouble or threats. (Also, "watch one's [own] back".) If someone is looking over someone elses' shoulder (ie. from standing behind), the person looking is trying to find out what the other person is doing.

It means that you are worried that something dangerous or unpleasant might happen to you, as when you look over your shoulder, you are checking to see if anybody is in pursuit. For example, whilst walking home in the dark in an empty street, you may look over your shoulder constantly.

Its a idiomatic expression means "keeping watch for danger or threats to oneself" cowboy...