The irony here, given the Alaska/FBI thread and the fictitious legal standard that you thought LEOs have to meet.
18 USC 111 generally applies if you've either ID'd yourself, or if you're on-duty but your status as a federal officer is still unknown to the assailant. So, generally, if a federal LEO is in plainclothes but working (to include UCs), they don't have to ID themselves before 111 applies...it doesn't even have to be a federal LEO for that matter, any federal employee that is assaulted on account of their duties is covered. The FLETC legal handbook covers 18 USC 111 on pages 156-159 if you want to google it (the PDF comes up in the first few options).