Honestly the only surefire answer is to lie and forge some paperwork, hire a contractor to rent a backhoe and ruin the asphalt, and then when the city stops you notify them of a sinkhole beneath.
They’ll dig and find whatever is there to find.
Yse as many cutouts as you can, but you’re already fairly exposed if this is true by your FIL’s actions. So have an alibi while all this is happening.