Determine Slab Thickness

[eaadams]

I had a tester tell me that if he doesn't know the slab thickness, he can take a small drill bit and drill through the entire slab. He says that he can 'feel' when he gets through the slab and then take that measurement AND he can do that without puncturing the vapor membrane.

Do others have suggestions on how to do this in a building where you don't have 'as builts'?