Ok, the easiest way to tell if someone is "low" is to watch their actions. If you know the person well, you will know if they are behaving "differently". By differently, i mean in an irrational way. For example, I was filling tanks one day and instead of opening the tank valve, i started fiddling with the rubber tank CAP (lol). My buddy saw this and knew i needed sugar, since what I was doing made NO SENSE. Lethargic (slow) actions are another sign.
ALWAYS GIVE THE "OK" SIGN WHEN IN DOUBT AND GAUGE THEIR REACTION. A DIABETIC MAY GIVE THE "OK" SIGN BACK OUT OF INSTINCT, BUT YOU SHOULD BE ABLE TO TELL THE DIFFERENCE BETWEEN A HYPOGLYCEMIC DIABETIC AND A "DISTRACTED" DIABETIC. If they don't make sense with their actions, end the dive IMMEDIATELY.