You wouldn't pay attention to oxygen saturation, and I'm not sure if you can even use that as a metric. Generally this involves heart-rate variability, heart-rate recovery, etc. This detects the stress on the immune system, which is detectable soon after your body starts dealing with an illness. If the asymptomatic case has a stress on the immune system, which it likely does, then this test will detect it (so it actually isn't asymptomatic in a strict sense). It won't be able to tell you that it is Covid 19, but it can be used as guidance to behave as if you have the asymptomatic case. You could even just look at your performance: a significant drop in output means your body is unhappy about something. Imagine running/biking/swimming/etc at full capacity when you have a cold: you can't.