It's easiest to find when you use a toy that is curved for g-spot stimulation; the toy that helped me mind mine was the Lelo Ella.
Then you can try rubbing the head against different spots at the front of the vaginal wall (facing your belly button). There will definitely be a spot maybe a few inches in that will feel better to have a toy against.
There are also different ways to hit your g-spot- like with a hard or soft toy (for more or less pressure). You can move a toy up and down against it, or in a circle, or just rocking with the toy inside you.