I find myself agreeing with Chicago on this, which I hate, but it is basically impossible, without torture or truth serum, to know whether the player knew he was hit or not. The only thing for certain is every single players answer to this question in a game environment, which will of course be:
"no, I had no idea I was hit"
or even
"get the xxxx away from me you xxxxing xxxx ref before I xxxx you in the xxx, you suck!"
If a player is found to be playing with a hit its a 141 (I would prefer a 241). If the ref see's him hit before he plays on then he could be pulled without penalty. If the player thinks he may be hit he can ask to be checked at any time, not that they will of course (in most cases) but they could if they weren't so busy trying to lose the hit as soon as possible by any means at thier disposal.
It seems strange to me that such leniancy should be displayed to players who have been hit when there is little or no fair play left in the game at the top level. I think penalties have to go up, and become far more severe (eg. minimum 241 for playing on) if the sport is going to be turned around and become something more palletable than body-building, which is a sport where you know full well ahead of time that the winners are the guys that cheated the most.