Ladies, (and gents, I suppose) this does not need to be tested. In fact, if you know what you're doing, you can actually physically feel the difference between g-spot (bundle of nerves at the base of the pubic bone) and the rest. Ladies, if you think you might not have one, it's never been hit. Fellas, if you think there is none, you're terrible at sex, or at least manual stimulation. Seriously, it's not some myth or magical spot that's indiscernable from the rest. It's there, you just need to hit it, deal with it.
And Holly has the basic premis down.