Lets say a wizard casts a compulsion spell on someone. Is that person aware that a spell was cast on him? Is he aware of where the spell came from?
A creature knows when it makes a successful saving throw against a spell.
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.
In addition, a creature can identify a spell after having rolled a saving throw against a spell by making a relatively difficult Spellcraft check.
DC 25 + spell level | After rolling a saving throw against a spell targeted on you, determine what that spell was. No action required. No retry.
That is, as far as I know, all we have on gaining information about spells that are cast on you, just by virtue of the spell being cast. You could, of course, notice that a spellcaster is providing verbal, somatic or material components, or directly observe the effects of the spell.
To answer your specific questions, I think we can infer:
- Should the character succeed on a saving throw against the enchantment, it knows that a spell has been cast. A character skilled in Spellcraft can recognize the effects regardless of the outcome of the saving throw.
- No, not unless the character noticed the spellcaster casting the spell or the spell has definite, observable qualities (most Enchantments don't).
I would say that the answer to this question depends on at least two things:
1) The target's previous experience with magic: If the target has no knowledge of or experience with magic, they are unlikely to look for a source for their current predicament. They are more likely to attribute the event to more "natural" causes like drunkeness, insanity, their OCD, etc. Even things external to them like a floating tea cup are more likely to be interpretted as indicating the presence of an invisible creature than a "mage's hand"
2) The necessary components (Verbal, Material, Somatic) for the spell employed: If the target suspects magic and the spell being used requires line of sight as well as incantation and a lot of arm waving being done by the only other sentient being in the room, they are likely to know who is casting. However, a wizard who has taken meta magic feats to cast without these components located in a crowd may be harder to identify.