Have someone turn the clock on and off while a zone is on but the water turned off. You should here a clicking noise in the yard. Be patient and look first around where the water comes in the house."The clicking is the valve solenoid"

I've had good luck finding buried valves with this tool.
You should be able to get one at a local irrigation supply house.

Here is a link that might be useful: Chatterbox

http://www.metrotelcorp.com/cableh.html

Cable locators can be found at most rental stores. Follow the wire.