On another thread there is a debate on how there is really no in-game mention of the armor cap. Here is my suggestion to give a normal player notice of the cap.
A status effect pops up on your bar saying something to the effect of: "Your armor is as strong as it can be, and cannot go any higher with subsequent skill additions."
(1) I use Armor of Earth and get +60 AL. The status effect pops up.
(2) I use Watch Yourself for +20AL. Nothing happens. Then while the shout is still up I use Armor of Earth for +60AL. The status effect pops up.
Pretty self-explanatory, but helpful.