From the first Google result for your question:

"A diagnosis requires evidence of degeneration of brain tissue and deposits of tau and other proteins in the brain that can be seen only upon inspection after death.

Some researchers are actively trying to find a test for CTE that can be used while people are alive. Others continue to study the brains of deceased individuals who may have had CTE, such as football players.

The hope is to eventually use a range of neuropsychological tests, brain imaging such as specialized MRI tests and biomarkers to diagnose CTE. In particular, imaging of amyloid and tau proteins will aid in diagnosis.

"Researchers are actively working to develop PET markers to detect tau abnormalities associated with neurodegenerative disease in people who are living.

Researchers are also working to develop tracers that target and bind to tau buildup and other proteins on PET scans. Studies are underway using these types of scans and tracers to look for tau buildup in the brains of retired athletes who experienced head injuries."

Chronic traumatic encephalopathy