How do you know your balloon isn't full of helium?
You could measure the force with which the balloon pulls up, then subtract the force with which the same ballon pulls down when filled with the same volume of air. From that, you can calculate the mass density of the gas that's in the balloon. Finally, you compare that to what you expect based on the known mass densities of Helium and Hydrogen. (Helium is denser than Hydrogen but less dense than air.)