That one's easy. How do you know when you get to Disney Land? Or to the Moon?
The goal of Buddhism is not an attainment, not something that you gain and so have to ask yourself if you actually have it or not, it's the elimination of all suffering through clear discernment of reality. Since this is a Theravada forum, that means the four stages of awakening (stream-entry, 2nd path or once-returner, anagami or non-returner and arahant). In the first stage, the delusion of self is shattered, leaving one completely sure that the Buddha taught the truth. The second stage weakens anger and greed to a degree that they are a mere nuissance. The third stage cuts away anger and greed completely, taking one beyond this world even while still alive. The final stage destroys all delusions; there are no longer any causes for suffering to arise within the mind.
Following the path, cultivating wholesome qualities and destroying unwholesome ones (i.e. right effort), and a good samatha-vipassana meditation can lead the mind to clear knowing, which results in the mind letting go... ceasing to grasp at the ungraspable.