I went to a GP. They referred me to a sleep clinic. I had a consultation then a referral to the test. The test was basically a data logging pulse ox meter that was mailed to me. I wore that overnight and sent it back.

They then recommended a CPAP.

In my case, it was just sleeping for one night with some diodes or whatever attached to me. Was able to detect when it flared up, very easy diagnosis.

My dentist of all people was the one who ordered it, but you may be able to go through other docs (I assume you can).