Easiest place to start is completely digging out around the valves and piping. Once everything is exposed you can determine if the leak is on the incoming side (coming to the valves) or on the outgoing side (after or at the valves). Run the sprinkers through a program at 2 minutes each and watch as each one comes on, watching for water seeping from the valves or piping. Once the leak is identified you can go from there on how to resolve it.

Thanks Steve, but my leak is through the inside of the valve

Gringo,
You can ordinarily hear water seeping through a valve that is leaking through the heads. If you know where the valves are, listen for a hissing sound from the valve that is leaking. Then to confirm your findings, turn on that valve manually at the valve via the manual bleed to be certain that it is the one in question.

All Wet
Texas Licensed Irrigator #1897 since 1984

Thanks ALL WET but my leak is so slight that I can't hear anything. I know by the loss of pressure in my well pump and tank that water is passing through one of them.
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Gringo,
How does this leaking valve manifest itself other than losing pressure at the tank? Do you have heads that constantly leak? If so, you identify that valve by manually bleeding it at the valve itself. If there aren't heads leaking all the time, you probably have a leak in the mainline.