Sprinkler System Problem

I just moved into an old house that has a built in sprinkler system. Unfortunately, one of the zones does not work, yet 2 of them do.
By researching previous posts, I've found some good ideas on how to check if it is the wiring by hooking up the wires from a zone that works to one that doesn't and testing it that way.
However, if it still does not work, apparently it is probably a valve problem. The thing is, I didn't know that each zone had a valve and I have no idea where a valve would be or what it would look like. I've looked everywhere near that zone for some sort of cutoff switch in the ground but can't find anything.
I'll be calling a sprinkler repair man if necessary but I'm trying to avoid it.
FOLLOW UP: For those with sytems... how long and how often should I water? I'm in Texas so the lawn requires frequent watering (I would think) but I'm also trying to keep my water bill at a REASONABLE level.
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