How do I know if I have to file a CA state tax return?

You are required to file a California tax return if you either worked or lived in the state of California at some time in 2012.

Even if my state income tax was only $7?

If that is how much your state tax liability is, then, yes.

If you do not file, I sincerely doubt the California Franchise Tax Board will come looking for you, but you DO have a legal requirement to file.

For CA filing requirement read this: https://www.ftb.ca.gov/forms/2012_California_Tax_Rates_and_Exemptions.shtml