Generally when mobs are enraged they get a bit bigger (similar to when hero is used) and sometimes have a bit of a red glow effect. There will also usually be a buff on the mob to indicate they are enraged. All you need to do is target the mob and cast soothe for it to work.
There are some mobs that normally gain an enrage buff, or this week, if you do a +4 key or higher, any trash mob under 30% health will be enraged and can be dispelled.