I am in a fresh water area, so do not have a first hand knowledge of saltwater issues.
In fresh water, if an anode gets a coating over the anode, it is no longer working...thus replace it.
In salt water, I would assume that the anodes get eaten more quickly...and if it is pretty eaten up, then replace it.
If it has a coating, you would think you could just wire wheel the coating off. No a good practice. When you wire wheel, you introduce parts of the wire wheel into the anode, then you have two dissimilar metals, ideal for electrolysis.
Changing anodes is easy to do, as long as you can get the bolts loose. To get to the anodes in the exhaust tube, you have to remove the covers to get to the anodes inside the tube. Be sure to get new o ring for the covers when you put it back together.
The anodes on the gear case are only held on by one bolt/nut.
The anodes on the stern bracket are held on by two bolts.
Again, the hardest part is getting the bolt/screws out and, for us old guys, getting down on the ground to take off the stern bracket anode.
Mike