In yesterday's game against the Giants, Lions tight end Dorin Dickerson sustained a concussion while covering a kickoff return. (It appears to have been on a simple touchback.) He stayed in the game, until he eventually screwed up on back to back plays.
Normally, we'd show you the play, and talk about the inextricable nature of violence in football, because, well, it's true. But here, we can't, because it happened offscreen, and because the play ended in a fair catch, there was no reason to show it. And Dickerson didn't take a knee, or ask to come out of the game. He just kept playing. Who could know?
Theoretically, the answer should be either a spotter in the press box with access to replay video, or the unaffiliated neurotrauma consultant present at every NFL game. These are specialists in place to observe the game under the assumption that players are going to lie, because players are always going to lie if it means they can stay in a game. But they can't see everything, and they're still—somewhat necessarily—keying in on the big hits, not every player getting knocked to the ground in kick coverage (even then, they miss some egregious ones). And certainly not every player making routine missed plays like Dickerson in the video above.