Is the unknown citizen fulfilled? If not, how do you know this and what poetic device is Auden using to make that point?

Probably the point of this poem is that the state, represented by the speaker, doesn't give a second's thought as to whether or not this unknown citizen was fulfilled. Fulfilment, or happiness, is simply not a measure the state uses to assess the life of a citizen.
Nonetheless, it is possible to find evidence from this poem to argue the point either way, depending largely on the meaning, or criteria one ascribes to "fulfilled." If, for example, one defines fulfillment as having a secure job, a family, good health, and friends, then yes, the unknown citizen can be said to have been fulfilled, for he had each of the above.
However, if one defines fulfillment as the opportunity or ability to fulfill one's potential, or as the opportunity or ability to be free and independent, then it is likely that the unknown citizen was not fulfilled at all. His life was lived according to an established set of criteria, and didn't seem ever to transgress the parameters established by the state. His life was, as the speaker says, "normal in every way."
By presenting the life of the unknown citizen from the point of view of a disinterested bureaucrat, Auden deliberately makes it difficult for the reader to determine whether or not the unknown citizen was fulfilled. The point is that we are not really meant to know, because, from the perspective of the state, represented by the disinterested bureaucrat, it really doesn't matter whether the citizen was fulfilled or not. And the fact that it doesn't matter, is the point that Auden is trying to make above any other.