Help me understand dopplegangers

[darthkilmor]

If an NPC the PC's meet for the first time is in fact a doppleganger, would they get any sort of sense motive/spot check to tell(assuming they have no previous knowledge of this npc)? What if they interact with him several times?

When a doppleganger uses Detect Thoughts, should the PC's even know they make a will save or should it be done behind the screen, so to speak?

And finally, are there any template's that mesh well with a doppleganger?

 

[Selganor]

Assuming that the doppleganger put some effort in his disguise, I don't see any reason why the PCs should ever doubt that the person they meet isn't the one he's supposed to be.

If people would be able to notice that someone was trying to read their mind, then Detect Thoughts would be quite useless, wouldn't it?

 

[Dalamar]

Characters know that they succeeded on a Will save, they do not gain any knowledge when they fail it. They also don't know what was the cause of the save.

 

[darthkilmor]

Characters know that they succeeded on a Will save, they do not gain any knowledge when they fail it. They also don't know what was the cause of the save.

Is this conjecture or from the PHB or RotG or ?

Edit: Nevermind, found the relevant section of the phb:
Succeeding on a Saving Throw: A creature that successfully
saves against a spell that has no obvious physical effects feels a
hostile force or a tingle, but cannot deduce the exact nature of the
attack. For example, if you secretly cast charm person on a creature
and its saving throw succeeds, it knows that someone used magic
against it, but it can’t tell what you were trying to do.