In Pathfinder second edition, does a spellcaster know the result of an enemy's save against their spell?
The game rules make it clear that when a player makes a check, they don't always get to know the DC they're testing against:
Sometimes you'll know the DC and make the comparison yourself. Other times, you might not know the DC right away. Swimming across a river would require an Athletics check, but it doesn't have a specified DC—so how will you know if you succeed or fail? You call out your result to the GM and they'll let you know if it's a success, failure, or otherwise. While you might learn the exact DC through trial and error, DCs sometimes change, so asking the GM whether a check is successful is best. (Player Core, p401)
However, neither the defenses against spells section nor the rules for making saving throws say exactly what the character causing a save should learn. Spell saves are generally not tagged as secret checks, but I'm not sure that would apply anyway (since it's an NPC making the save, not a PC).
In my experience at the table, the degree of success on the save has always been made clear by the GM (so that I can roll the right amount of damage or describe the other effects of the spell to them), but a fellow player suggested this may not be the Rules as Written.