Well, I'm not expert on MLF, but I think there are some signs. The most tell-tale is the reduction of malic acid. If you don't have a chromatography kit, then it's more of a guessing game.
If you're thinking that MLF didn't even start, then that may be correct. If your sulfite levels were fairly high, for example, MLF might not get started even if innoculated. Here's some interesting info from winemaker magazine: WineMaker Magazine - Malolactic in Action, Lowering pH: Wine Wizard -
I didn't have a chromatography kit, so I just did a waiting game. I waited about 9 months, and then just assumed it was finished. It tasted better (less malic acid) to me, but again, it was just a guess and not the "proper" way to do it.
Now, I would NOT recommend a MLF on cider. Yes, the apples are mostly malic acid, but that is what gives it it's flavor. If it happens naturally, then it does, but the cider would be more "flabby" tasting and less sharp, like cider should be.