Hard to say. There was a discussion on my sportbike forum a while ago about sugar in the tank, and there was a large group of people, myself included, who wondered if sugar would dissolve in gasoline. You would think it does, but solvancy of various compounds don't always follow common sense. Worse scenario, someone dumped 20 pounds of sugar in your tank, and if there is enough dissolved would probably foul your plugs, carbonize your combustion chamber, and otherwise gum up the injectors over time. Best case scenario, someone dumped a cup or 2 in there, and the sugar dissolves slowly in your tank, and you see no ill effects. I invested 20 bucks in a lockable gas cap, maybe you should too.
If you are really paranoid, as I would be, siphon out as much gas as you can, drop the tank and clean the thing out with clean gasoline. Also, CHECK YOUR OIL PLUGS too, they may have been loosened up/taken out. (Made any enemies recently?( Maybe check the filler neck with your finger to see if you can't feel and granules in there.