GFI Receptacle Tester

I picked up GFI receptacle tester that tests standard and GFCI outlets. What a great tool to have on hand. I have a question about using it on GFCI outlets. There's a black button on the tester that instructions say to push IF testing GFCI outlets.
So do I plug the tester in to the outlet, and take the reading while i'm holding the button down? Do I take the reading after pushing the button? My question sounds trivial but I'll only feel confident knowing exactly what it's telling me. Thanks to any whom can help!

 

Plug it in and read the lights to see if the receptacle is wired correctly. Then push the button. If this receptacle is GFCI protected, AND IF THIS RECEPTACLE HAS A GROUNDING CONNECTION, then the GFCI will trip and all the lights will go out. Then you'll need to go reset the GFCI (hopefully you can find it). If nothing happens when you press the button, then either the receptacle is not GFCI protected, or you have no grounding connection.

 

In anyone's 'expert' opinion, are these things worth having (GFCI testers--I'm not talking about the GFCI receptacles ) ?

 

Absolutely. For $8, these things are really cool. I think there should be one in every toolbox, even if you do no electrical work at all.

They can sometimes be a bit misleading, but I can overlook that. For example, they often report "hot/ground reverse" when you have an open neutral. And they can sometimes cause people to think they don't have GFCI protection when really they just don't have a ground. And if somebody wired a bootleg ground (i.e., connected grounding and neutral in an outlet somewhere), they can give you the false security that you really have a ground when you don't.