Does the AI have a mechanism to know somebody engineered a war against it?

The Gods & Kings expansion introduced the concept of "Sharing Intrigue" which allows the AI and you to share bits of information you gather about each other's plans. It's certainly possible that Venice somehow got word of your plot and that was the source of the denunciation. When you're talking to them, you can hover over the word that describes their attitude toward you (ie, "Guarded") to see reasons why they think you are a good or bad person.

There's another factor here, though, and that's that the AI will become increasingly suspicious of you as you close in on victory. If you're in the Modern era, chances are good that you're working towards victory at this point. If any AI figures this out, they are likely to become aggressive towards you.