For me it was a combination of very deep meditation that pointed at It and then the waking equivalent of a lucid dream experience that cemented it. My meditation practice wasn't enough, probably because I barely "touched" the lower level(s) of Samadhi. I'm not Sri Ramakrishna and the 1,000 - 2,000 hours I have practiced is just a tad short of the mark. LOL!

I suspect in essence my experience showed me something very similar to what your NDE showed you, perhaps the exact same "knowing" and that is of the non-dual reality. Even though it's now a very subtle background reverberation it's still there and something that seems like it can't be "unknown".

Disclaimer: No, I'm not Enlightened but I did have a profound Awakening experience that, while it lasted, seemed like Enlightenment. To be totally truthful I was at first absolutely crushed as it faded into the background and took the better part of a year to come to a fuller understanding and thanks to Advaita, which at the time I had only dabbled in for a month or so before my experience.