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Zone of truth states:
On a failed save, a creature can't speak a deliberate lie while in the radius. ... An affected creature is aware of the spell...
Does "affected" here indicate that creature failed their save, or that they are within the radius of the spell? Put another way, is a creature aware of the spell if they succeed on their saving throw?
RAW, it doesn't say one way or the other. However, I interpret the spell as creating a zone in which not telling the truth becomes more difficult. If you fail the saving throw, you can't overcome the effects of the spell. If you succeed, however, your willpower is sufficient enough to overcome the effects of the spell. I would personally rule that a creature knows they are in a Zone of Truth after they step into the circle and either succeed on or fail their saving throw.
Spell Description on page 289.
Page 179 are the rules on saving throws. In it, we're told that a creature can either save or not save against an effect. So, each spell/ability/effect must provide two effects: A save effect and a failed save effect.
Only one of the two outcomes is provided, the one for a failed save. By the rules for saving throw, you only become aware of the spell once you're subject to that affected area. Thereby, you can't actually know the spell is there until you fail a save (Specifically, you're told you become aware once you fail). This is because the opposite situation (effect of a save) is not present, and RAW doesn't exist - IE you don't know its there if you saved since your character doesn't know what a saving throw is.
The catch is you need to make sure nobody sees you cast this for it to matter. The spell has both a Verbal and Somantic component, meaning there's words spoken and a gesture made for the spell to be cast. So, if the npc in question is a spell caster and knows the spell there's a chance they could identify it via the Arcana or Religion skill and gain knowledge that way.
While I can't find any RAW at a quick search, affected typically means that the spell effects are active on that target, in this case it would mean they've failed their save.
Interpreting this to the specific spell, consider that if you were in a Zone of Truth and were trying to tell a lie, but were unable to (because you failed your save), this would be pretty obvious to you what's going on.
Meanwhile, if you were in a Zone of Truth, and had succeeded at saving against the effects, if you tried to tell a lie, you would be able to, so you would not necessarily know something was odd.
In either case, if you're trying to tell the truth, you wouldn't really know you're in a Zone of Truth. (It might be argued that no matter how had someone tries to tell the truth, there is always a hint of a lie to it, minor embellishments etc, that the Zone of Truth would make its effects known still, had they failed their save and tried to tell "the truth")
To be "affected" by a spell is not the same thing as to be "targeted" by it, and the rules for spells consistently use these terms in a distinct way.
For zone of truth, you are only affected by the spell when you fail a save against it and are then forced to be unable to lie. It's the only situation in which the spell subjects you to an effect, which is literally what it means to be affected. Since nothing at all happens when you succeed on the save, succeeding doesn't make you affected by the spell, so you won't automatically know anything about it.
To put it another way, if the creature were supposed to know they were subjected to the spell even if they succeeded on the save, the paragraph would start with a "targeted creature," not an "affected creature," because the former wording would apply to any creature subjected to the spell while the latter wording would be restricted to only those under one of its effects.
However, there could be other reasons for a target who succeeded on the save to know that they were being subjected to a potential spell effect. For example, they could notice the sudden glib honesty of those nearby who failed the save; they could be familiar with the spellcasting components enough to recognize it during the casting; they could be under interrogation and simply recognize through context that a spellcaster is trying to manipulate them. Nevertheless, none of these edge cases grant the automatic knowledge one gets when failing the save.
