AFL club doctors have a lot more data on their players as well. Amongst other medical data, they conduct flat line tests on the players periodically to identify any potential issues.
For example, with Angus Brayshaw they would do some testing with him after he was drafted so that they had initial flat line details available. Then, after each concussion, the doctors would have done more testing and assessed when he was back to that flatline and he could resume.
As the number of concussions increased, this flat line would have been higher due to his past issues and that would have been taken into consideration to medically retire him because those new flatline levels were of concern to the medicos and the league.
The issue for Brayshaw was that his last lot of brain scanning identified significant changes from his previous round of tests. The drs could clearly see damage to his brain that wasn’t present before