If you're in a business, train your team to ask strangers "Hi. How can I help you?"
The wording of the question is important. "Can I help you?" gives the person the opportunity to say "no." The word "how" quietly forces the person to state a purpose. If he/she is unable to answer this question easily, that is a tip-off that the person could be up to no good. Be polite and personable but don't let strangers go unchallenged.
Make sure you teach your team what to do when someone strikes them as suspicious. If the person claims to have forgotten an ID badge, have him/her escorted to your main entrance to be properly signed in. Likewise, if the person claims to be law enforcement, escort him/her to your Security Office so that credentials can be verified. Remember, badges can be faked.