Apologies if this is a silly question, but everything I've seen seems to devolve into arguments without a solid answer on this..
If something has Permanent Enlarge Person cast on it, and someone casts Reduce Person on them, does that just instantly dispel the Enlarge effect or does there need to be a caster/dispel check?
From FAQ "...it doesn't matter if the target would normally get a saving throw or spell resistance to negate or avoid the spell used to dispel (such as casting slow to dispel an already-caste haste); to speed up gameplay and prevent lopsided applications of this sort of dispelling, the "diametrically opposed" spell automatically dispels its opposite, regardless of the desires of the creature affected by the opposite."
In instantly dispels, no save or check.
Illuminating the Darkness paizo blog contains the following:
To dispel, you simply cast your spell on the same target (just like with enlarge person and reduce person) and then they cancel each other out, leaving no spell.
So official position seems to be no checks, it just works.
If you cast it on yourself, it MIGHT be protected by permanency, but "Spells cast on other targets are vulnerable to dispel magic as normal", so otherwise, no dice.