Recognizing classifying toposes Suppose $\mathbb{T}$ is a geometric theory, $\mathcal{E}$ is a topos, and $M$ is a model of $\mathbb{T}$ in $\mathcal{E}$.  Is there any sort of elementary condition on $M$ and $\mathcal{E}$ (or, even better, on the geometric morphism $\mathcal{E}\to \mathbf{Set}$) which would allow us to recognize $\mathcal{E}$ as the classifying topos of $\mathbb{T}$ and $M$ as the generic $\mathbb{T}$-model therein?
I feel like this is a long shot, but I thought I would ask anyway.
Edit: Of course, such a condition could not be expressed in the internal logic of $\mathcal{E}$ (even including non-geometric logic), since then it would be preserved in all slices $\mathcal{E}/X$.  This is one reason I feel it's a long shot; but the example of principal bundles mentioned in the comments suggests that it's not an entirely unreasonable question.
 A: This is several years late, but it may be helpful nonetheless. 
As alluded to by Buschi, Olivia has given an explicit answer to this in Theorem 2.1.29 of her monograph Theories, Sites and Toposes:

Let $\mathbb{T}$ be a geometric theory, $\mathcal{E}$ a Grothendieck
  topos and $M$ a model of $\mathbb{T}$ in $\mathcal{E}$. Then
  $\mathcal{E}$ is a classifying topos of $\mathbb{T}$ and $M$ is a
  universal model (i.e. generic model) of $\mathbb{T}$ iff the following
  conditions are satisfied:
  
  
*
  
*The family $F$ of objects which can be built from the interpretations in $M$ of the sorts, function symbols and relation
  symbols over the signature of $\mathbb{T}$ by using geometric logic
  constructions (i.e. the objects given by the domains of the
  interpretations in $M$ of geometric formulae over the signature of
  $\mathbb{T}$) is separating for $\mathcal{E}$.
  
*The model $M$ is conservative for $\mathbb{T}$, that is for any geometric sequent   $\sigma$ over the signature of $\mathbb{T}$, $\sigma$ is valid in $M$ if and only if it is provable in
  $\mathbb{T}$.
  
*Any arrow $k$ in $\mathcal{E}$ between objects $A$ and $B$ in the family $F$ of condition (1) is definable; that is, if $A$ (resp. $B$)
  is equal to the interpretation of a geometric formula  $\phi(\vec{x})$
  (resp.  $\psi(\vec{y})$) over the signature of $\mathbb{T}$, there
  exists a $\mathbb{T}$-provably functional formula $\theta$ from
$\phi(\vec{x})$ to $\psi(\vec{y})$ such that the interpretation of
  $\theta$ in $M$ is equal to the graph of $k$.
  


I'm not sure if you were looking necessary and sufficient conditions on $M$ and $\mathcal{E}$, or just merely sufficient conditions, but since this Theorem gives an 'iff' result, one might try and prove the sufficiency of certain (perhaps more intuitive) critiera on $M$ and $\mathcal{E}$ by checking against the conditions listed in this theorem, i.e. by proving results of the flavour: 'If $M$ and $\mathcal{E}$ satisfy condition $X$, then they satisfy the 3 conditions of this theorem.'
Extending this thought, I am curious to see how these conditions relate to the special case mentioned in Dylan's comment. In particular, how does weak contractibility relate to the conditions spelt out by Olivia? This is not obvious to me, but I haven't taken the time to properly work through the details.
