I don't think there is anything in the rules covering this, but the 3.0 FAQ did have a question on it.
MainFAQv06272003 said:
Being polymorphed doesn't change your identity - you're still you. Scrying effects tuned to your own identity can still locate you. They also can locate you when they're keyed to assumed identity, too, depending on the scrying character's knowledge of you.
... Someone who knows the imposter only in his assumed identity can scry for him and locate him in that identity.
That FAQ answer still leaves some ambiguity but here's how I'd run it based on this answer.
1) The scrier knows the true identity of the impostor: he can correctly target the impostor with scrying no matter what form it is in.
2) The impostor's assumed form was duplicating a real identity:
2a) If the scrier does not know that the impostor was anything other than it claimed to be, then
i) he targets the real identity if he has met it or knows of it, or else
ii) he targets the impostor if it is currently in the assumed form of the real identity, or else
iii) the scrying attempt fails as the intended target does not exist at the time of casting.
2b) If the scrier knows the identity it met was an impostor, then
i) he can choose to target the real identity, which may fail outright unless the scrier has an adequate connection to the real identity, or
ii) he can choose to target the impostor in the assumed form of the real identity, which will fail unless the impostor is in the assumed form of the real identity at the time of casting.
3) The impostor's assumed form was a fictional identity, regardless of whether the scrier knows the impostor was not what it claimed to be:
i) he targets the impostor if it is currently in the assumed form of the fictional identity, or else
ii) the scrying attempt will fail as the intended target does not exist at the time of casting.
I think that covers all the bases. In all cases that can result in failure I wouldn't distinguish it from a successful save against scrying. The scrier would simply know that the scrying attempt failed for some unknown reason.