Wise-seeming people have been saying for a long time that the whole world is a dream (or illusionary in some way) and that you (as the privileged truth-seeker) have been given the means to penetrate right-down to the real-deal. What you need, they all say in one way or another is the cure.
In Inception, it seems pretty clear that the cure Cobb used to wake his wife up from their dream-world didn’t quite work out the way he planned it to. Instead of a cure, he infected her. Instead of saving her life, he inadvertently caused her death.
So, truth seeker, how do you know that instead of being cured, you’ve actually been infected?