Taken from California Law:
The headlamps and every light source in any headlamp unit
shall be located at a height of not more than 54 inches nor less than
Whenever a requirement is declared as to the mounted height
of lamps or reflectors, the height shall be measured from the center
of the lamp or reflector to the level surface upon which the vehicle
stands when it is without a load.
^^^^One way they can determine if your vehicle is too low.
This is generally how most states determine if your car is too low...
24008. It is unlawful to operate any passenger vehicle, or commercial vehicle under 6,000 pounds, which has been modified from the original design so that any portion of the vehicle, other than the wheels, has less clearance from the surface of a level roadway than the clearance between the roadway and the lowermost portion of any rim of any wheel in contact with the roadway.
^^^ So in other words, if any part of your frame is lower than the lowest part of your rim, then your car it illegally low.
Here's a great link in determing if your car is legal in California or not... http://www.dmv.ca.gov/pubs/vctop/vc/vc.htm
Just click on the letter of the term that you wish to be interpreted. For example, to find the answers that I found above, I clicked on "C" for Clearance. If you wanted info about how loud of an exhaust you can have, click on "E."
If you still don't know how to use the site, just let me know and I will kit you over the head with a tack hammer because you are a retard.