I always identify the exits and make a note of potential cover. Many restaurants have large mirrors that can allow you to monitor spaces behind you. It is possible to do all this and not be a boor. ;-)
LOL! I know, you really can’t have everybody lined up in the restaurant with their backs to the wall; there’s only so much wall space.
As you suggest, a mirror is a good solution. But of course, most people don’t notice what’s going on around them anyway, so maybe the very first thing they need to learn is vigilance.