If a given individual has Glibness pre-cast and someone attempts to use Discern Lies on them, do they then know if they pierced some kind of effect (if successful) or are aware that they failed to pierce some effect (if unsuccessful).
Since it's a targeted effect that allows a will save, the caster would know if they passed that save.
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. Likewise, if a creature's saving throw succeeds against a targeted spell, you sense that the spell has failed. You do not sense when creatures succeed on saves against effect and area spells.
So far, this has been the only relevant information I can find (I haven't even been able to find a reference for whether you'd know if you failed to pierce SR or not), even though this interpretation would nerf the spell quite a bit (they can't tell if you're lying, but they know you've got a spell effect up to help you lie).
As a GM, I'd probably rule it that if they fail to pierce the effect, it feels identical to if you passed your will save. They'd know you resisted, but would not be aware of how you did so.
However, I'm very interested in hearing other interpretations. Maybe someone knows a rules reference I've missed.