

Nick doesn't actually have super strength.Instead Grimm's powers work by weakening the Wesen who come into close proximity with them ala anti-magic which is accompanied by a supernatural sense of dread. It doesn't totally drain the most powerful ones like Ogre's but it does even the playing field a bit and Wesen notice which is how they realize he's a Grimm. This is why we never see Nick perform any of the typical stuff associated with superhuman strength. Unless you count that time he punched through a window but people in TV land can do that without super strength anyway. In "Over My Dead Body" Monroe explained to Hank that Wesen's have two types of Woges - one where they barely shift when under stress or afraid, and one where they fully shift intentionally.

Grimms - and only Grimms (or other Grimm-types from other cultures) - are able to detect the first kind of Woge, which is their "superpower." When a Wesen intentionally Woges, that's when anyone can see them (and it result in them going mad, or reports of monster attacks leading everyone else to think they're mad, etc.)

They can tell Nick is a Grimm only when he sees them turn.They knew he was a Grimm because he reacted to them "losing control".
