Does trained mages' chance to hit depend on Accuracy or Spell Mastery?

That's a good question, I have no idea, but I would like to know too.  Talking about magic staff wielding troops, right?

Affirmative.

They don't ever seem to miss.  I might be way off, and I don't know for sure, but they don't seem to miss.  Now, to be fair, the last group I created were from a well upgraded fortress and a faction with lucky, so they had a starting accuracy of 120ish I think...so I can't tell if it's because they would never miss anyways, or if mine are just so skilled.

I haven't bothered looking myself, but I imagine you could infer which it is from the chance to hit and damage range you get from the mouse-over  popup when it is your mage unit's turn to attack.  Look at the chance to hit, and check the enemy's dodge/spell resist vs. your accuracy/spell mastery.

I hope it is accuracy, because I'll feel silly having given all my mage units the precision trait

I believe it's accuracy. Spell Mastery should matter when determining the chance to resist the spell effect.

As far as I know, the ranged elemental damage attack from staffs (pariden's special staff "Leht Staff" included) is an accuracy vs dodge attack.

Sincerely
~ Kongdej

I know it's accuracy, because my dodging heroes dodge them the same way they dodge everything else.