When is a polynomially growing automorphism of F_n geometric ?

The main result of this paper is an algorithmic answer to the question raised in the title, up to replacing the given $\hat{\phi} \in Out(F_n)$ by a positive power. In order to provide this algorithm, it is shown that every polynomially growing automorphism $\hat \phi$ can be represented by an iterated Dehn twist on some graph-of-groups $\cal{G}$ with $\pi_1{\cal{G}} = F_n$. One then uses results of two previous papers \cite{KY01, KY02} as well as some classical results such as the Whitehead algorithm to prove the claim.