Sprinkler System Help

I've been running my system a lot because we just overseeded the lawn. I think that I may have an underground leak in one of my zones. The area in question is really wet, all the time. It's next to the sidewalk, and that is always wet too. Almost like the wetness is coming from under the concrete.
I'm pretty sure I have a leak, but can't see compelling evidence of it. (like water peculating up through the ground) Does anyone have a good way to find or check for leaks without digging the whole thing up?
Thanks, Joe Michel
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