I can only comment for my girl. When I use my fingers on her g-spot I feel a rippled section an the top of the vaginal wall just past the pelvic bone. When she's not aroused - there's nothing there because it's made up of erectile tissue that engorges with blood with sexual arousal - like a clitoris. There is also erectile tissues in the vulva that cause them to engorge as well.
Back to the g-spot - once aroused her g-spot seems to be about 2-2.5 fingers wide and about 1-2" long.
Not being a woman I'm at a disadvantage here - but it seems that body geometry would make self-discovery difficult. My wife has never felt her own g-spot - but I have no problem finding it, stimulating it or creating an orgasm through it.
As my wife says - I don't have to see it believe it - I know it's there!