There is a way to monitor this in theory.

A spy close to a settlement will tell you all the pertinent information about it, relative to the province it is in and this includes income at a settlement level, and a province level on the province tab (though this is hugely innacurate more often then not if you don't have a clear set of information about all settlements in the province, and as the AI never gets big enough for corruption to be a major issue it is usually much higher than you will actually get.)

These numbers are pretty accurate at a settlement level and, in theory at least, should therefore tell you if the province in exempt from tax. I have never seen the AI do this at all, ever.

In responce to Bramborough's point about AI and provinces, I disagree. Carthage, for example, has loads of settlements that are the only one in that province they control at the start and ,if coded correctly, could happily exempt one settlement without losing a large amount of income and have that settlement still produce the right troops ect. This could work, but ATM the AI doesn't use it.