Fruit Beer

I brew a peach wheat that I add roughly 12 pounds of peaches to. I add some at primary (pasteurized of course) and some at secondary for extra flavor. I pee,l mash, and pasteurize all the fruit my self. My question is how do I get an accurate OG reading after the fruit has gone  in? It's obviously going to raise the sugars but I have no idea how to measure it... Please help ???