In most cases, income, marital tax status, and age determine whether a taxpayer must file a tax return. Other rules may apply if the taxpayer is self-employed or if they are dependent on someone else. There are more factors to determine residency. For more information, see publication 103 of the Guidelines for Determining Resident Status (FTB).
If you're still not sure if you should file a tax return, ask a tax professional, call the IRS at (800) 829-1040, or make an appointment at the nearest IRS Taxpayer Assistance Center.