There are tests (which should be adminstered by independent physicians and not team personnel) which cannot be faked to determine if a player has a concussion. But neither the league nor the teams want to do such a thing.
All the "those guys knew what they were getting into" arguments get thrown out the window if the league can be shown to be negligent. And in order to show this, we need look no further than last December when Colt McCoy got absolutely blasted by James Harrison. Everybody watching that game could tell he had a concussion. So what happened? He was back in the game a few plays later. If that's not an example of gross negligence, I don't know what is.
That was one of the most irresponsible things I have ever seen in professional sports but, of course, for all the empty words of player protection, there were no consequences to the team for doing it.