Once you get to know the people around you, you will be better prepared to notice strangers who seem to be hanging around a lot. Seeing someone you don't know walk down your street once is not at all unusual, but if that same person were to be lingering and going back and forth on your block frequently, they may be casing the neighborhood and looking for a home to rob.
Similarly, if you note an unfamiliar car that is spending a lot of time driving on your street, it could be an indication that criminals are looking for potential targets. Warn a neighbor if you see someone looking at their home, and don't be afraid to call the police to report suspicious behavior. As it is often said, the best way to avoid being victimized by crime is to prevent it from happening.