Polymorphed NPCs and / or enemies (disguise)

[Joesi]

I performed a search, and while difficult to find accurate terms, I surprisingly could not find any discussion about this.

Many creatures or NPCs in a game world may use polymorph spells to appear as something else. A good example of this would be a succubus, dragon, or a [humanoid or any intelligent race] spy.

My question is about what circumstances would players be able to notice that. Sure something like detect magic would obviously work unless it was masked, but I was specifically thinking more about using perception.
It brings up the issue of dealing with paranoid players as well, although that's not specifically what this is about; it's just curiosity.

Any polymorph effect counts as a disguise with a +10 modifier, and has penalties for being a different race or size (for example). Theoretically this would mean something like a dragon would be relying solely on it's disguise check to be disguised.

That said, even if it's disguise was terrible, the rules (disguise skill) seem to state that they'll get away with it unless the creature does something strange/remarkable (or unless they're disguised as someone that someone's seen before). This would be GM's discretion I suppose, but what are you guys' opinions on this?

Perhaps something I said is also wrong? Regardless, this is a hybrid discussion between both advice and rules, and I'm quite curious about it. Everything about the rules for this subject just seems strange, vague, or even outdated. Anyone have any better rule suggestions? (house rules)

[Nefreet]

I've always run that it's the player's job to specifically ask for a Perception check vs. Disguise, and (as you've said) it's only the paranoid players (not characters) that regularly "ruin the fun".

Those are also the types that walk around town "disbelieving everything", and that collect every iota of mundane gear to sell for gold. At that point it just becomes a matter of whether you want them to continue in your campaign.

Whether it's a home game, or a PFS table, taking the player aside and talking with them about their problem, and how it's potentially dragging down the experience of the other players at the table, may be all you need to do to remedy the behavior. If your whole table is that type, well, it'll probably continue to be a problem.

Mechanically, it's just a Disguise check with a bonus. The polymorphed creature can take 10, and I don't see why they couldn't take 20, so with a little aid only the most Perceptive of PCs should notice that something's amiss. A hooded cloak could give another +2, distance could play a factor, and maybe they received the Aid Another action from their Familiar when polymorphing ("Hey, boss, not so long in the nose!").

Hopefully, if you can roleplay and describe encounters with NPCs skillfully enough, Perceiving vs. Disguise won't be on their mind.