Also I'm now intrigued: Jack will use it on apparently normal looking people...

The thing is though, Jack didn't use the mind probe until after they'd discovered that Beth's skin couldn't be broken.
At that point he knew they were dealing with (best case) some dotty scientist who'd installed a force field on a human or (worst case) some alien who was pretending to be human.

Either way, Beth was definitely not 'normal'.