For example, does the caster of the spell remove curse know if the subject rejects the spell?
It might be an unusual event, but this just happened in my game, and we couldn't find the answer.
For example, does the caster of the spell remove curse know if the subject rejects the spell?
It might be an unusual event, but this just happened in my game, and we couldn't find the answer.
Yes, the caster is aware.
Casters are aware of whether or not a target has resisted a spell via 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.
Furthermore, all beneficial spells with the harmless tag are resisted with saving throws, not out-right:
Harmless: The spell is usually beneficial, not harmful, but a targeted creature can attempt a saving throw if it desires.
Still, consider that it is often unimportant to go into the details of the caster being unaware and confused by this decision. It may not be worth your time to explore this through roleplaying unless it serves some narrative purpose. If there is some interesting drama to be had by the target of the harmless spell keeping there motives for resisting secret, consider playing that out in the game; if it adds nothing to the game, just skip over it.
The Player's Handbook says on Succeeding on a Saving Throw says, "[I]f a creature's saving throw succeeds against a targeted spell... [the caster] sense[s] the spell has failed" (177).
The 4th-level Sor/Wiz spell remove curse [abjur] (PH 270-1) has as the entry Target: Creature or item touched, so that spell's caster knows if the subject succeeded on its saving throw.
(Note that the subject can't just outright reject the spell and is entitled to a saving throw against the spell despite the spell's harmless nature, the harmless tag on the spell's saving throw meaning only that "[t]he spell is usually beneficial, not harmful, but a targeted creature can attempt a saving throw if it desires" (PH 177).)