You activate it when Grimm are close. It affects every Grimm the pawn can see.

how do tell if has characters has silver eyes or get a characters with silver eyes

You can see that in the pawn's health tab, where something like bionic eyes would be.

I can't find a button to trigger it though

Your pawn needs to have Aura or a Semblance unlocked. Then it is right next to the Aura status.

Hypothetically, if someone was horrible enough to capture an enemy pawn with silver eyes, and then surgically remove them, would it be a bug if the surgery was a success and still did not generate an eye "part" (silver eye or regular) or would that be intended?

It is intended to not create silver eye parts.

How do you get them though?

It is random and they are really rare. You cannot really plan to get them.

引用自 Carny Senpai：

It is random and they are really rare. You cannot really plan to get them.

what about thought prepare carfuly?

If you can give your pawns a silber eye hediff to each of their eyes that should work.