No, minor dehydration will not cause damage. If you stayed severely dehydrated during the day, your symptoms may be worse.

You would not know you have OSA. Others would notice your breathing irregularities while you are asleep. You may wake up often during the night. Some people do not wake up fully. Just enough to change positions and start breathing better. People with sleep apnea are often tired during the day and can easily take a nap.

There are recording Pulse Oximeters that you can wear on your hand and wrist that tracks your oxygen saturation as you sleep. They cost $35 to $150. Google recording pulse oximeter.
A FitBit will tell you how often you move during your sleep and if your pulse varies.