I believe it is based on reports from the different police departments. I don't think Trulia gets the information directly from them, but that is the original source of the data. I have talked with a few police departments over the years and they classify and report their crime statistics based on type of crime. The best way to find out about a specific area is to call the local police department and ask to talk with their community liaison. I have found that they do a pretty good job in sharing information.
I hope this helps.
JOSE DIAS, REALTOR