How can that be known except through self-reporting? At least with lucid dreaming, experiments have shown communication from the subject via eye signals[1]. Has a similar thing being done with meditators?
[1] http://www.historydisclosure.com/scientific-experiment-prove...
I'm sure to some this sounds bogus or like a cop-out. But experiencing it (or similar states, as can happen with psychedelics) can impact our waking lives in meaningful ways, so I hope it's not brushed under the rug for much longer.
If meditators remember it then it probably has physical manifestation in whatever mechanisms brains use to remember stuff. Maybe some day it will be possible to observe this being stored in the brain.