Short of spending a million dollars right then and there, you can use historical market data. Or datasets provided by the person you're trying to prove it to. Keep in mind that after a certain volume, your model would start influencing the market itself.
Some markets can absorb an enormous amount of volume, such as the FOREX market.
Now, when you start making good trades in a "big" market, intelligent players can mimic / play off of them, so it is hard to prove in that regard.
At a minimum though, you could prove that it works on assets that are not volume-dependent.