I know the clarifications about knowing whether you were yourself affected by a spell, but I can't find information on whether the caster of a spell knows if the target makes their save or not.
The specific situation that brought this up was a suggestion spell where it wouldn't be obvious to the caster whether or not the target had been affected. I ruled that the caster knows (because it was a PC and I temporarily rule in PC favor when in doubt), but I'm not at all sure that is correct.
Zone of truth specifically says that the caster knows, which implies that the caster doesn't normally know.
Has there been a clarification that I'm unaware of?
Highly technical answer: It depends, and it's the DM's call.
What I do: Most spells have some means of being obvious as to whether the target made a saving throw. Damage is "less", the target dives out of the way, the target is not whisked to another plane of existence, concentration is not in effect, etc.
suggestion is one of few spells where the effect may not be obvious. I usually roll out in the open, but one time a PC used
suggestion against a war cleric they had just knocked out after a very difficult fight (players were starting to chant, "TPK, TPK!"). They woke him up and used
suggestion to have him get them into the neighboring cult so they could assassinate the leader. This time I hid my roll and rolled a natural 20. He agreed with their plan and succeeded against insight checks to see if anything was going on. He led them through the enemy encampment, and when he felt he could escape he yelled that the party were intruders who needed to be killed. The party survived and shot the cleric from across a gorge as he was escaping. He fell into the shallow gorge and took a death saving throw failure but rolled a natural 20 on his next death saving throw and got back up. The party was furious, started spending their resources to hit him, and (some) even jumped into the gorge to make sure he died this time. It was vastly satisfying to pull the players in like that.
tl;dr
suggestion is, in my opinoin, one of the very few spells where there is no RAW/RAI indication of whether it worked. A Wisdom (Insight) check can fix that, but sometimes the story is better served by the party not knowing.