According to the NHL's Concussion Evaluation and Management Protocol, a player shall be removed from the ice and taken to a distraction-free environment for concussion evaluation if, among other things, he has a blank or vacant look or is slow to get up. If the player complains of symptoms such as feeling slowed down or not feeling right, he must undergo evaluation.
These are subjective observations and symptoms. Furthermore, the protocol acknowledges such symptoms are not necessarily unique to concussions.
As for the evaluation, part of the process is for the player to complete the Sport Concussion Assessment Tool � 5th Edition (SCAT5). Components include recalling the date, repeating a five- or 10-word series and reciting the months in reverse order. It is a comprehensive test.