There's no way to "prove it", considering there's hardly a difference between a pirated version of the tf2 beta and an official copy, except maybe a hurr durr u got this from csmania or something.
Aren't supposed to have it != can't have it. I'm sure valve realized they let the public run loose with SFM, but they obviously don't care considering they haven't said even one word about it. I've probably sent multiple emails to different employees at valve about SFM, whether they're aware or if they're fine with it, and none of them got a response. Either valve wants to stay neutral about the argument, or they just don't really care.