Private security guards can't use the word police on their uniforms. I looked it up a while ago (I asked the exact same question you did) - IIRC the rule is that you are only allowed to use it if you aren't issuing any orders and there isn't an unrelated reason for people to think you are an official officer. If you see the word police they are either someone pretending to be an officer (and must follow the rule I just wrote) or a real police officer.
In many cities (including mine, which has a lot of private security) the word POLICE is written in all caps and large letters on the back and front of their shirts. It's hard to make the distinction until you see them once, then it's pretty clear