When someone records you in a game and accuse you of wall hacking, then the GM's will check the video to make sure you weren't falsely reported (people get reported for actually being good at SF).
Then they watch your movements and if you manage to kill someone without any possible way of hearing them or seeing them through a window or door or something then they suspect you of wall hacking and they take affirmative action from there on out.
That's how I would do it at least. They can't take a screenshot of your computer before someone sends in the report.
Besides, I used to have an account strictly for hacking and I was never banned regardless of how obvious I made it look that I was hacking. IJJI and Aeriagames usually just patch the hack and let the rest fall into place until another hack breaks through.