If 6 years had passed with limited sensory input, and you were pretty sure you remembered all of it, how would you know if that's true?

How much detail do we really remember of our own last 6 years of fully awake, conscious existence not spent in such a state?

I feel like once you get to such a length of time you can't trust your own memory of it. Obviously he could know enough to say "I was conscious during some point of that time window" but measuring it would be hard.

I remember the movie about LIS.. I couldn't bear to watch it. 6 years of it is beyond insane.