Your best bet would be to contact the local police department or try http://www.city-data.com. They have some useful info on crime and other stats for various areas in the US.
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There is a lot of good websites that you can use as tools to determine the crime rate. Another good tip would be to contact the Phoenix Police and they will be able to give you numbers on the crime rate.
I always recommend that in addition to checking out the diffrent websites you get familiar with the community your interested in. Make sure you drive by in the morning, mid-day, evening and weekends. This goes for any community you're interested in. Good luck!
You can check the links already mentioned and also call the police department for further information. You can pick up homes for low prices in Maryvale just make sure to check it out thoroughly.