Hi Geoff, welcome to the board.
I'm not highly versed in California Special controls but I'll try and help until someone else with more knowledge comes along.
Not all California Specials have the same controls, but I'm guessing you've got two knobs and three switches, and no pickup selector.
The tone control is a low pass filter. Open all the way, full clockwise, all frequencies are allowed to pass (up to 6khz); as you roll it back, you begin lowering the threshold above which higher frequencies are rolled off. Closed all the way, you should only be hearing low frequencies, around 60hz. So it should be very noticeable.
The three switches should be on/off switches for the three pickups. The two that are three way may have a treble boost in addition to on/off. Remove the control cover on the rear. You should see some little blue square boxes with white round adjusting slots on top; these are trim pots. If there are more than two, then you may find that two of these are in proximity to the two three way switches; if so, then these two pots control the amount of the treble boost.