The only difference between "I did it because I wanted to" and "I did it because my enemy put a gun at my head" is in my memories... and that part is lost.

Physiologically speaking, there is in fact a difference--and this is a difference you can use.

If, for instance, your precautions include measures like sending yourself blood samples, it's likely that you'd observe significantly higher stress hormone levels in your blood if you were coerced than if you were doing it freely. So if you measure high levels of stress hormones in the blood sample, you can be fairly sure some sort of coercion was involved.