ToME: the Tales of Maj'Eyal

Is there any kind of built-in function I can use to protect the player against any kind of "field" or "cloud" effects, such as glacial vapour or the old acid fumes one?

If not, do they have some common identifier that I can use to make my own to check against? I suppose I could do the long way and check if damage type is GLACIAL_VAPOUR and any of the other ones but I'm hoping there's an easier way

You could superload engine.map:processEffects to set a flag somewhere letting you know it's in the middle of doing overlay effects.