I got a cheap Xiaomi Smart Band 9 (for less than £30) to get a feel for my pulse/oxygen saturation while I'm sleeping with the CPAP. Napping without the CPAP, my saturation was about 90%. With the CPAP on, my saturation was 98% or higher. I think that's pretty conclusive evidence. It's also quite a cool little device for the price, even if I did have to buy a much bigger strap to fit it onto my gorilla arms.
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In order to qualify for having sleep apnea your SpO2 must be **below** 90% **and** you must have a disruption in your breathing. At 90% you would be borderline and this would not at all be "conclusive" but at best suggestive. Keep in mind anyone without sleep apnea would see an improvement in SPO2 on a CPAP, so the improvement isnt an indicator either.
If you want conclusive evidence you will need an actual sleep study.
