When do i know if my alternator is going bad in a 1998 Lincoln navigator?

i have had to replace my alternator 3 times due to my system :D but the first sign is usually your interior lights dimming and flickering when you are at a stop. such as your miles before empty gauge that would always catch my attention. and yes this is when your at night and your headlights are on. and you will hear your alternator constantly whining. i got a 250amp aftermarket one with a capacitor which solved my system problem :D