Checking with the local police station and visiting the area at different times of the day and different days including weekends -- all good advice even if you think you know the area. Also, below most listings in Trulia you'll find information about reported crimes in the area. In the "Local Info" section click on the "Crime". If you zoom out you can get an idea how this area compares with other areas of the city. Also as mentioned there are other apps and tools. The San Francisco Police Department uses crimemapping.com. Use the link below, scroll to the bottom of the page and click on "I agree".
Prudential California Realty
1715 Polk St., San Francisco, CA 94109