If you find the G-Spot on a girl, and she enjoys it, you will know it, especially the first time. Her eyes might suddenly become huge, she may tilt her head back, or gasp for air. Either way, you will most likely know you hit it after a few seconds, say 20-30. Either that or she will suddenly tell you to stop pressing there, it makes me have to pee. Whatever the sign, you probably have it.