Usually that sort of thing is a distance. Take a tape and swing an arc 20 ft from the left corner and 19 ft from the right corner. The cutoff should be where the arcs intersect.
If the arcs don't intersect or it doesn't make sense, then you need to try something else. A metal detector is usually a big help.
Be aware that you will probably not have a tool to operate the city cutoff valve. They often require a five-sided wrench to operate, to prevent tampering.
You should have an inside cutoff near your water meter.
