When you see them in the room it says "The High Guard stands here" or something like that. But when you try to trip them you get "They are already down."

They should be standing, as per their desc.

Also, many of the high guards have names that speak of specific things, i.e. someone Oakenshield. He does not have a shield of oak. Might want to just do a runthrough of the mobs in the area for that sort of thing.