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You would normally use a metal detector, to locate the valve under the floor, other options are intersect the pipe and us a flexible rod or endiscope to work out location or distance of the pipe run before the blockage/valve.
I have also seen mixed success using a thermal imaging camera as the pipe is often colder than the floor.
However most of these things are very time consuming, the quickest route is bring up one tile create an inspection hatch and then locate it, then lift the tile which is above the valve.
Let me know if you have any further questions about locating the valve.