ZDoom

That does sound like a good idea. I should have thought of it myself.

I'm not going to say that the functionality couldn't be useful elsewhere, but I feel like you should be able to remember what sky was last set yourself already (store the string if you're generating it at run time? or the index if you're looking it up from a big array) and do the comparison, rather than having to ask the information for something you can keep track of yourself.

I already knew about the LevelLocals struct. I was hoping to avoid ZScript for this.

The bare bones is: define a function in ZScript that does the check, and then call the function from ACS using the ScriptCall ACS function.

(Gotta leave home for work soon, so no time to flesh it out right now; but hopefully that should give you the ideas to research)

I worked out a day/night system for a potential future GZDoom mod. The problem is that changing the sky texture every second seems to slow down the framerate. I think that if I could do a check for the current sky texture it might speed things up. I'm not sure though so I'm asking. I know ZScript has this feature but I can't see any good way to integrate this into my ACS.