It should be pretty easy to have some algorithm on the server that puts a tag on the title of a thread when it is necrobumped. Say, a new post is recorded 6 months after the last post -- server automatically puts "[Bump]" at the front of the title. Anyone scanning the list of threads sees it is an old thread resurrected.
Second this idea if possible... Have found myself two pages into a necrobump before deja vu kicks in and I realize I read it all ten years ago.