What would be the most common key signs that a person with sleep apnea need to be woken up?

Stop breathing : The key signal is stoppage of breathing. An apneic will usually exhibit a rhythmic (if not obnoxiously loud) snoring pattern. When they exhale and begin to snore on the inhale again, the tissues of the tongue and airway collapse like a straw in a thick milkshake. Because they haven't taken a breath, their co2 levels are high and they struggle to get a breath. Get them to a sleep doc asap.

Lack of Breathing !: If the person makes absolutely no breathing sounds, and then starts "fighting" to catch a breath, this would be a good time to wake them up. No one ever had apnea when they were awake. This person must have a sleep study (at a clinic or at home) and a determination needs to be made if CPAP or oral appliance will successfully treat them. Apnea, if untreated leads to significant medical issues.