with the amount of digging required to "pothole" the head it would be cheaper and quicker to just find the main line near the closest head, tee it off and run a new line. But capping and running the zone should give you an idea. I had one job I was brought in on, a client had his lawn regraded and the landscaper was suposed to raise the head. The head was never raised and was buried under under 9-12" of loam and sod. We capped the heads and ran it for about 45 minutes before we were confident on what section of sod to tear up, we narrowed it down to a 4-4' section and ripped out the grass. After that we were able to track down right were the head was.