You may have noticed that each real estate agent has his or her license number shown on his business cards, website, ads, etc. This is mandatory. Somehow who does not want to give you their license number is most certainly not registered. Do not do business with such!
Now, if you may be wondering how to check that the person is really using his or her license number and that the number is currently valid. This can be done by going to the California Bureau of Real Estate (CalBRE) and entering the license number.
The current location of that information is as follow:
This page includes a form that you can use to search by license number. The result is a page with the full name, address, and current type of license (salesperson, broker, and any specialization if relevant.)
We always suggest that you verify that the people you are dealing with have a license which is current.