First look for depressions and ditches, like Sprinkus said.
Here, I've had good results noticing different grasses and weeds growing where the valve depressions are. Even after many years of growth, the spot looks different. Look for any anomalies.
I'd be interested to know what 1idejim's method is for locating hydraulic valves with a locator. I've heard of people who have pushed piano wire in the black tube and traced it. I have not done that.
What I have done is to "pot-hole" and count control tubes. For example, if you know that you have five zones and you dig up five tubes, you know that all the valves are all ahead of you. I like to find a halfway mark. Guess. Keep digging and when you find there are four tubes in the hole, you know one valve is behind you. Keep going.
Also, Wet_Boots mentioned a cut tube. Before you start digging I would look for any fence post, planting, phone lines or anything like that. And/or any new digging.