I've heard that when you're talking to someone you can look at their ears and they won't notice... Unless you tell them. But it might not work too well if you're within so many feet or inches from their face.
But the bridge of their nose might be fine too.
Ooh! No. Keep looking at whatever it is their other/straying eye is looking at. Stop for a minute and look in that direction. After the 3rd or 4th time walk over to that area and closely inspect, then look right back at 'em and give them a confused glare while turning your hands up and forward in a 'shrug' like motion... Then slowly resume what you were doing. Then, at any random time, rush over there where they were looking again and yell "Ah-HA!" When you notice nothing is there again point at them and give them that glare again.