Is it possible to become enlightened without realising it? My question is prompted by the thought that if your progress is very gradual and you do not have a measuring stick (or any labels), you might, after years of meditation become enlightened but only realising it when you read the works of enlightened masters writing about their own experiences.
according to the early Buddhism doctrine codified in the four Nikayas of the Pali and Agamas of the Chinese Tipitakas that is impossible
realization of nibbana is necessarily accompanied by the knowledge of destruction of taints or knowledge of liberation
Through dispassion his mind is liberated. When it is liberated there comes the knowledge: ‘It’s liberated.’ He understands: ‘Destroyed is birth, the holy life has been lived, what had to be done has been done, there is no more for this state of being.’”
Because of being disenchanted, he does not delight in it. Because of not delighting in it, he is liberated. Being liberated, he knows and sees: ‘Birth for me has been eradicated, the holy life has been established, what had to be done has been done, I myself know that there will be no receiving of any further existence.’
It is possible, @Uluru, but not for you and I, nor anyone else. The short answer to your OP is ONLY those beings who are destined to be Pacceka-Buddhas (Solitary Buddhas) will fall into this category.
Solitary Buddhas are born in the years just before a fully enlightened Buddha appears. We are still at the early stages of the present eon in which one other Fully Enlightened Buddha will come when a time comes of a people with an eighty-thousand-year life-span. This will be after a long period of spiritual darkness blankets the world, that these individuals will eventually be born who, through their own effort, re-discover the long-forgotten path to awakening and liberate themselves once and for all from the long round of rebirth, thereby becoming Solitary Buddhas. They lack the requisite development of that of a Supremely Enlightened One. So they are unable to articulate their discovery to others and that is why they are known as a "Silent" or "Private" Buddha (paccekabuddha).
Becoming a Pacceka Buddha is not easy. In one’s previous life one has to perfect generosity to the level of relinquishing of one's children, wives, and belongings, such as wealth, and relinquishing of one's own limbs and even one's own life, and had heard the Supreme Dhamma of a Fully Enlightened One at least in one lifetime to become a Pacceka Buddha.
There is a very simple answer. How did Buddha know he attained the enlightment? Didn't he realize it? When you become enlightened, your heart will tell that to you. You will feel it. So you will definitely realize it when you have attained Nibbana. As the last step, when you are just about to attain it, you will give up the "requirement of attaining Nibbana" as well!. So there you are!