My Apple Watch has an oximeter (I bought it before that feature was pulled in the US), and there's a "sleep apnea notifications" section in the "Health" app, but I'm not sure if the absence of notifications implies an absence of a problem.
Would sleep apnea always be visible as a lower blood oxygen level when sleeping, or can there be negative effects even without that?
Ringconn, similar to Oura without the compulsory subscription, claims to be able to detect sleep apnea with 90.7% accuracy. see https://ringconn.com/blogs/news/understanding-sleep-apnea-an...
> Would sleep apnea always be visible as a lower blood oxygen level when sleeping, or can there be negative effects even without that?
It would not always be visible, no, if your AHI was low enough. It is still damaging even if your blood oxygen is okay, because you are waking up to keep it that way. You're not getting enough sleep as a result.
You might try the recording trick I mentioned below to listen for sounds of it, but if you're unsure at all, I'd consider seeing a sleep doc.
If you're concerned with Sleep apnea testing is very easy these days. You goto a sleep specialist, they give you a little kit you take home, you put it on for one night, they look at the data. Done! It's not like even a decade or so ago where it's "goto sleep lab", most of it's done at home now for the initial investigation.