A diode does not stick. It's a one way valve for electrical current flow. When broken it's either open or shorted (flows either way). Usually when it breaks it's normally open.
There are 6 diodes in the 3 phase bridge. An oscilloscope will tell if the alternator is not putting out clean electricity. The alternator rebuild shops will have an oscilloscope. AutoZone or AAP associates will not know what an oscilloscope is.