Claiming to be enlightened by itself doesn't mean that it's not true. It's just that even during The Buddha's time when He was alive and teaching there weren't that many arahants in the world (maybe a few thousand or so). Let alone now in modern times, long after the pure dhamma has disappeared. An arahant is rare.
The Buddha had mentioned teachers and beings who falsely claimed to be all-knowing and enlightened.
It is possible for someone to achieve higher states but not the ending of mental fermentations and mistakenly believe themselves to be enlightened. It is also possible for non-arahants to achieve iddhi powers and many feats.
If I were to encounter someone who claims to be an arahant or fully enlightened I would ask them this simple question "How do you distinguish arahantship from achieving higher states without achieving arahantship?".
Although I myself can answer the question I wonder how the arahant claimant would respond.
But I believe The Buddha was who he claimed to be, not simply a paccekabuddha but a Sammāsambuddha, meaning both fully enlightened independently and a supreme teacher because:
- No other teacher in all of recorded history ever debated with so many people of different views and explained things in such a fearless, doubtless manner
- No other teacher in all of recorded history ever reasoned things in the ingenious way that The Buddha had
- No other teacher in all of recorded history ever explained things in the detail that The Buddha had
- My personal experiences matched in closely to what The Buddha had said
I've read stuff from other spiritual teachers and those who claim to be enlightened but still they don't compare to Gautama Buddha's teachings. Some may be paccekabuddhas but their teachings are limited and aren't as well-reasoned and thought-out as Gautama Buddha's.
My personal experiences match in closely to what I read from The Buddha. When I concentrate I can accomplish certain things, like healing myself. I believe I've achieved at least something like the first jhana but am not sure if it's exactly what The Buddha described as the first jhana since he describes many different states (see Pancakanga Sutta).
I just know that when I concentrate on destroying painful feelings or fermentations that I feel this strange energy go from my forehead to the top of my head, and if I keep trying to eliminate more fermentations I go higher and higher. What I experience then is a happiness unlike any other.
I feel doubtless, fearless, sorrowless, angerless, full of confidence, calm, unstoppable, full of enjoyment, such an extreme type of enjoyment.
Once I had experienced this type of enjoyment I thought this was the correct path and no other form of happiness, bliss, or enjoyment was worth pursuing.
It is entirely different from the other blissful states I had experienced (like deep relaxation, or the dream-like bliss) which are like blissful states but lacking energy and enjoyment.
Maybe some day I'll find out everything and the whole truth...