trouble is that you could have made the case for a Real Estate bubble in 1999 and would have missed out on the last 3-4 years of appreciation. Tops of bubbles are notoriously difficult to call.
I would look for a pop in ST interest rates back to the 2-3% range as a the trigger. You would then expect to see listing inventory climb. Inventory climbs first and only slowly do sellers begin to lower prices. You would also expect a increase in the 30 day delinquency rates. Dataquick News provides monthly updates. That number is still low.
previous booms in California have lasted about 4 years: 1975-1979; 1986-1989; 1996-2000; 2002 to 2005/06??? the next year will be very telling, IMO