How can a person know if they have sleep apnea without a sleep test?

Typical symptoms. Symptoms of obstructive sleep apnea (osa) include snoring, witnessed pauses in breathing, snort arousals, feeling unrefreshed upon awakening, morning dry mouth, daytime sleepiness, tiredness, memory/concentration difficulties among others. Physical features such as being overweight, set-back chin (retrognathia), large neck are also suggestive. You would need a sleep study to confirm.
They can't. There are symptoms, there are signs, there are scales. They all suggest sleep apnea, but the only way to prove it is an objective test, a sleep study.