I'm not sure what, exactly, the answer would be... but the only thing I can lend is that, for certain directors, I tend to see a similar style from film to film (first one that jumps in my head here would be Eastwood). I haven't watched Argo yet, but I'll instantly be looking to see how it compares, stylistically, to The Town (& perhaps it won't at all). I think that some directors have the ability to affect their films on the macro level, whether that's through instructing contributors like DPs or editors, or simply choosing something like what time of day to film an exterior shot (for an example). I'm sure you'll get far better answers, but the similarities I feel like I notice in Eastwood's films were what jumped into my head upon reading your post. I definitely notice similarities across movies too. Like every Wes Anderson movie for instance. The soundtrack, the long shots, even the credits. But if you take one individual movie and were to judge how it was directed, what would you look at? With some directors, you know what their style is and therefore what they are contributing. But for a relative newcomer, you probably wouldn't know what they contributed unless you worked on the film.