I don't believe that's possible (yet) but the good news is that several Research Institutes are said to be conducting on going research into diagnosing C.T.E while the person is still alive..
> At this time CTE can only be diagnosed after death by postmortem neuropathological analysis. Right now there is no known way to use MRI, CT, or other brain imaging methods to diagnose CTE. The CTE Center is actively conducting research aimed at learning how to diagnose CTE during life. Find out more about our research here.
Source: Boston University’s Chronic Traumatic Encephalopathy (CTE) Research Center => https://www.bu.edu/cte/about/frequently-asked-questions/
[1] http://www.nydailynews.com/sports/i-team/new-brain-scan-leap...
Of course, I may have misunderstood him and/or his opinion may not be in sync with the consensus in the field. Still, it's not something I've heard in these discussions.
Just thinking out loud, does anyone know if this could ever be diagnosed while the person is alive? I'm not very familiar with the disease but just knowing you have it while you're alive would give people at least an opportunity to try and do something about it before it's too late.