I addressed this after Hopkins' concussion back in 2013.
This is essentially the NFL Policy of identifying and managing concussions.
http://images.nflplayers.com/mediaResources/lyris/pdfs/NFL_Diagnosis_Mgmt_Concussion.pdf
In particular, this is how a player is tested for concussion initially, and for navigating the NFL Concussion Protocol.:
There are many baseline tests given to all of the players before the season begins. One of those important tests include the
IMPACT test [
CLICK HERE TO SEE THE ACTUAL TEST....THE SIDELINE ASSESSMENT TOOL.]. It is the tool used to first assess potential concussions right after they are suspected. Other tests are given that test colors, shapes and memory association combinations. There are audio and visual light tests given. There are numerous computer programs that the players are presented with to test reflexes and timed cognitive responses. These are all compared to the baseline levels/scores established during these tests given before the season began. This is all above and beyond the comparative physical and psychological testing performed by the team doctors and neurologist that are performed at regular intervals (the Concussion Protocol).
The process, if followed as intended, is so complicated that it is pretty well accepted that it would be virtually impossible to fake "recovery."