I Have Just Been Diagnosed With Sleep Apnea, I Was

I have just been diagnosed with sleep apnea, I was told it was mild most of the time but severe during deep sleep. I have never been told I snore, I am female, over 40 and overweight. I am sleepy during the day, however it doesn't interfere with my daily activity. I did have a sleep study done. How is it determined that I have sleep apnea and how am I sure it's a correct diagnosis?