Help with trim tab anodes

When you get the new ones, compare them to the existing ones. If they feel/look the same, then the existing are aluminum, as well. Keep the new set for the future since the existing ones still have plenty of life left.
Great idea!!!

Out of curiosity how do I know when they should be changed? I really have limited experience with this.
 
Magnesium best for fresh, Aluminum or Zinc for salt/brackish water