I don't know about private clinics but any public health center will test you; you can incur costs if deemed necessary i.e. meds....oh, one more thing, testing is anonyous.

I don't have first hand knowledge, but maybe this will help get you started.
http://local.yahoo.com/results?fr=dd-local-more&stx=std+clinic&csz=San+Diego%2C+CA

Another good resource, even if old fashioned, would be the SBC Yellow Pages.

I just got checked using an online service.  TSTD.com  You can go and get your blood drawn just about anywhere and the company arranges all the tests then you get the results in a few days.  It costs $300 and they do about 7 different tests.  It is anonamous.  If you are in this hobby, it is important and responsible to get checked out at least two times a year.  I have been in the hobby for two years and have seen over 70 different providers.  I have never gotten as much as a cold.  I am always safe and everything is always covered.  Enjoy.

There is a place in LA in the Valley on Ventura Blvd where the porn actors/actresses go.  I forget the name, but it should be easy to find with a google search.  Excellent service, no lectures, no problems.  Classier than the free STD clinics sponsored by the country and CDC.

I dated a girl with Herpes (she told me upfront) and 3-4 weeks after we broke up I had a small sore (about the size of an eraser at the end of a pencil) on the head of my favorite friend.  OH SHIT!!!  Told her, she was in tears, told me cying, that she always knew when she was infected and had never, never infected anyone else in the 10+ years she had been infected.

Anyway, I got the anti-body test for HERPES (and several other STDs, just for good measure) and everything was negative.  The sore went away after 5 days or so and never returned.  I never found out what it was.  Maybe I just rubbed against my shorts the wrong way.