Knowing which skills count as knockback can only be confirmed through testing, but see the list below for confirmed skills and some insight. At least for Monks, pulling monsters in counts as knockback (all runes of Cyclone Strike confirmed).
List of confirmed Monk skills that proc Stongarm --
- Fist of Thunder - Thunderclap
- Deadly Reach (knock-up)
- Lashing Tail Kick - Sweeping Armada
- Tempest Rush - Bluster
- Wave of Light - Wall of Light
- Cyclone Strike - all runes
List of confirmed Monk skills that DO NOT proc Stongarm --
I'm not sure about non-Monk skills, but this may give insight into Blizzard's wording on what they consider knockback. Source: Druin's monk mechanic workshop. At the time of posting this, the link's content is up to date as of 2.0.6 and later 2.1 PTR builds.