Hi.
How those testers know if the grounding connects to the breaker panel or is just hooked to the neutral at the outlet ? Or they do not ?

 

They don't. That's one way to trick them is to jumper ground to neutral.

 

Normally at the outlet, there should be in the range of ~500mv AC (neutral wire resistance voltage drop, could be higher or lower voltage but easily detectable) between the ground pin and neutral on a circuit in use with nominal loads. If they are connected at the outlet that should be near zero VAC. Those testers can't see it but a meter will.