How I can test that the ground cable of my AC power outlet is really grounded / correctly grounded ?

Is a quick test that I can do or an instrument that does just that ?

Also, how is grounding rated if it's rated at all. Some x amount of charge discharged per each second? 

I'm well aware that most buildings basically have a ground wiring for each power outlet, and all ends up linked to a metal stick in the ground, but how do I check the grounding when I'm near a power outlet and I don't know anything about the blueprints of the house or anything else ?