When the police receive a tip, there are two conditions that must be met before a police officer can use and act on the tip: The first is that the officer must have reason to believe that the one giving the tip is personally reliable as a general matter; the second is that the tipster is speaking from good information in a particular case. This is the standard for when tips are given and the tipster is known, but there is an entirely different standard used when the tip is given from an anonymous source.
When a tip is anonymous, the information given must be more than what can be casually observed. Meaning, if the tipster gives a description of the subject and his location, along with a tip that the subject is engaging in illegal activity, this is not enough for the police to take action because the subject's appearance and location can be gathered just by seeing the subject out in the open. While someone's appearance and location does not warrant a stop, it may justify one when combined with other evidence.
In a recent case, Commonwealth v. Saunders, information was given to the police through a Facebook message from the account of Brandy Mullins. Because the police were unable to determine that the tip-giver was, in fact, Brandy Mullins, the police used the standard of the anonymous tip to determine if there was enough information to make an arrest. The tipster gave the make, model, and color of the vehicle along with the description of the driver and passengers. While this typically would not have been enough to make an arrest, the tipster stated the road on which the vehicle would be traveling and an approximation of the time frame. Thus, the time frame and route would not have been known by the casual observer, so when the officers were able to verify that the tip was able to predict the future events of the vehicle, it became reasonable to conclude that the Facebook tipster had reliable information about the subject's illegal activities and a stop was justified.