There are number different ways that an experianced person can determine that
1- Check with The city of Santa Ana for Permit go in person
2- Go Public Record office City Of Santa Ana
3- Look at the other houses in the area.
4- Look for separate Gas, Water and Electrical Meters
The Most important of all is to check with the city and have an expert to determine it for you.
A unit is determined to be legal when all appropriate permits have been completed (signed off by the City). Ussually permits can be found in the City's website http://www.ci.santa-ana.ca.us/pba/ You may also stop by the City M-T to make sure you get all the details; just ask for any permits issued on (address) and they will let you see them right at the desk.They can get copies of the same for you at a low price.
City of Santa Ana requires a business license to rent a unit in Santa Ana. Annual dues are about $114 per property and you don't need to take a course to get the business license.