All a "pen like device" can do is test for the presence of electricity. It CANNOT test for a GFCI circuit, CANNOT test a GFCI device, but CAN test for polarity if the user know what he is doing, which in this case may be suspect.
Next time you have a inspection take the OSHA inspector out to lunch.
A OSHA inspector is never wrong.
Just like the Boss...
1. The OSHA inspector is never wrong
2. When the OSHA inspector is wrong, see #1.
The inspector most likely was testing polarity using a neon tester.
They do work with only 1 connection. (The Human Body is the "test" return path)
If polarity is reversed then the GFCI should trip if any load is put on that outlet. (Given the Ground is properly connected in the outlet.)
Without a GFCI the outlet could be a hazard. I think it could be wired wrong. No reason for the inspector to look like a DA, Chances are they will prove their case. They will Win for sure.
The Outlet should be Marked with a circuit number and GFCI warning but it is not always required by code.