Would repetitive football injuries to the cranium show up decades later, causing symptoms resembling mild retardation, OCD, etc.? What is the best way to determine this in terms of imaging, testing, etc.?
yes, here is a nice paper which shows evidence : http://www.nature.com/srep/2013/131017/srep02972/full/srep02972.html
from the paper : " a report commissioned by the National Football League (NFL), showed that retired players between the ages of 30–49 were 20 times more likely, at a rate of 1.9%, to receive a diagnosis of dementia, Alzheimer's disease (AD), or other memory-related impairments11. Over the age of 50, the proportion diagnosed with one of the above rose to 6.1%, compared with only 1.2% In the general population11. Overall, neurodegenerative mortality of NFL alumni is 3 times greater than that of the general population, with AD and amyotrophic lateral sclerosis (ALS) estimated at 4 times as high '
It's hard to say just, because it depends on the location of the head injury and how severe the head injury is. The brain is composed of several lobes which all have a different functions. Depending on the location and severity of the brain injury, the symptoms show up.
For example, when you get brain injury in the frontal cortex, and this is severe enough, a person can become somewhat antisocial. With antisocial I mean saying whatever they think of. This causes awkward social situations. The impulse brake is gone then.
Let's say you have a hemorrhage. The location of the hemorrhage is key to predicting which kind of symptoms you are going to show if the hemorrhage is severe enough. Some people who have had an hemorrhage experience a change in personality and likes and dislikes. Other people who have had an hemorrhage experience trouble in their motor skills, like dressing themselves.
To answer your question: it depends on the severity and location. A brain injury, repetitive or not, can cause a variety of symptoms, but you can't experience a mental illness like schizophrenia when you get brain injury, because the base genetic predisposition towards schizophrenia is already there when you are born, just like OCD. Brain injury which is not already there when you are born is called Acquired Brain Injury, or in Dutch, N(iet) A(angeboren) H(ersenletsel).
A neurodegenerative disorder that appears in boxers after repeated hits on the head is called Dementia pugilistica. I have heard some people call it punch-drunk disease. It manifests as Dementia but often involves paranoia. There is an article on wikipedia about it.