Enlightenment is best defined as a nondual perspective, you see you not in terms of the self, but rather as a integral part of the universe. It is typically achieved through a process of mysticism.
Mysticism is meditation, meditation is mysticism, they are the same thing, no daylight exists between those two things. Before one starts meditating, mind is cluttered, and holds on to certain delusions. Through meditation, those delusions are seen to be false.
The mystic path has a rhythm, and dramatic arc. If you’ve ever run into the concept of the monomyth, the same arc is at play in mystic meditation. You start, you gain power, you gain false mastery, which then leads directly to a fall, then you start dealing with the your shadow sides in the Dark Night, leading eventually to Fruition.
Achieving Fruition once does not make you enlightened. Instead it just starts another development arc, where you move through the mystic stages more rapidly and rapidly, experiencing and moving through Dark Nights more smoothly, until your mind no longer has any resistance to moving though the mystic states. This is enlightenment.
It is perfectly possible to be enlightened yet still be ignorant of the fact. The thing you want to pay attention to is both your mind, and the ease at which you move through mystic states. Your mind is always in a mystic state, but if you’re enlightened, then you can ‘push’ it to enter into a different one seemingly at will.
If you’re not enlightened, then your mind has an inertia that makes it really difficult to do any kind of meditation. Your mind is filled with random stuff, fears, worries, that stuff you promised your mom you’d do. It’s not loose and flowing like water.
My mind has been loose and flowing for as long as I can remember. When I finally started doing meditation, I found concentration much easier than most people describe. As soon as I realized it was possible to clear my mind, I just started doing it all the time. The limitations to mystic practice, for me, were always completely self-imposed. Whenever I could wrap my head around any spiritual or mental idea, it just started happening for me.
I was already neck deep into mysticism for years before I first had the inkling that I could be enlightened.
The concept I use to understand this stuff and how you can detect enlightenment in others who don’t do any mysticism at all is consciousness. I could write a book on consciousness, and I probably actually will one day, but the main idea here is that people vary in their ability to ‘push ideas around in their head’. This isn’t localizable to a single ability, like “intelligence” or even “wisdom.” Instead it’s a combination of all the mental abilities. And you can have more or less of it than others. People with more of it are closer to enlightenment than others.