When I took huiman sexuality in college we discussed the "G" spot. If she actually has one, try havig her lie on her stomach and insert your fingers into her vagina, with your palm facing down. It may be about 2 inches in, and feel like a small, smooth ball. BUT, the "G" spot, or Graffenburgh is something that may not even exist at all. Good Luck!!
This is something you have to find on your own with her. Every woman's G spot is different because every woman's cervix and vagina are shaped differently. Get a book on sexual touching and in there will be different techniques that can help you. If your wife is game get a "BOB" (battery operated boyfriend), and have fun with it, the right size one will help aid you in the finding of the spot. Don't think you're not big enough for it, it's just that you can move "BOB" around in there a little easier to find the spot.