If a creature succeeds on a saving throw against a targeted spell, the spell's caster only knows that the spell failed
I admit, that knowing only this is a lot like knowing if a creature that's the target of a spell fails or succeeds on the saving throw, but there are some weird wrinkles.
Magic on Saving Throws on Succeeding on a Saving Throw says that "if a creature’s saving throw succeeds against a targeted spell, [the spell's caster] sense[s] that the spell has failed[, but the caster does] not sense when creatures succeed on saves against effect and area spells." (That's for any spell with a Target entry that includes creatures, by the way, not only spells (and, by extension, many effects) of the school of enchantment.)
But this is terrible phrasing, and, I suspect, rarely used as written. For example, when a caster casts charm person and the targeted creature fails the saving throw, the GM says, "You sense nothing," but if the targeted creature succeeds on the saving throw, the GM says, "You sense your spell has failed."
So while a caster knows when creatures targeted by the his spells fail or succeed on their saving throws, that information is delivered in the most awkward way possible. It makes casting a targeted spell that has a saving throw like entering a sweepstakes that says Mail this entry form and, if you don't receive a reply in three days, you've won $1,000! or like a doctor saying, "If I don't call you in 24 hours, it's not fatal." In short, when a caster casts a targeted spell that has a saving throw, sensing nothing about the spell is the good result!
However, it appears the designer of the story feat Twisted Love uses this rule like I suspect most use this rule. The feat's completion benefit says, in part, that "if [the creature possessing the feat] succeed[s] at a Will save against… an [enchantment] effect, the caster does not learn that you succeeded at your saving throw." So it seems to me that the idea is that when a creature that possesses this completion benefit succeeds on a saving throw against a targeted enchantment effect, the benefit is supposed to change what the caster learns from The spell failed to nothing. However,— technically,—normally a caster already learns nothing about the subject's saving throw and, instead, learns only if the targeted spell has failed, making a strict reading of that part of the Twisted Love feat's completion benefit itself do nothing.
Of course, were a GM actually to make such an twisted, highly technical argument—when what the feat's completion benefit is supposed to do is incredibly obvious—solely to deny my PC that completion benefit of the feat Twisted Love, I'd leave that table, and so should you.
Note: The discern lies spell does provide the caster with spell failure feedback as described above as it's a targeted spell, but the spell zone of truth normally does not provide the caster with such feedback as the spell's an area spell. This means when a zone of truth spell is hitched to a hallow or unhallow spell—depending on the zone spell's caster—none, some, or all of the creatures that enter such a hallow or unhallow spell's effect may be affected the attached zone of truth spell. Hilarity, I'm sure, ensues.
