^I believe banned members can still log in, and their profile continues to record such an action as "Last activity" (Their abilities/permissions around the forum is just very limited), so if there hasn't been any sign whatsoever of a log in, it is likely that the member just hasn't visited lately
As for knowing about banned members...usernames are not published for obvious reasons, of course, so the best suggestion is probably to click on the "email ____" link in the member's profile and contact them directly regarding a situation
