Many smartwatches have the hardware to measure oxygen saturation, though not all have it enabled. For example, all Apple Watches have hardware for it, and rumo

Many smartwatches have the hardware to measure oxygen saturation, though not all have it enabled. For example, all Apple Watches have hardware for it, and rumors (from before Covid was declared a pandemic) suggested that the next OS version would enable it. Depending on your worry level, you might want to wait to see what happens. Also, this article mentions that people are subconsciously altering their breathing/heart rate when affected. If you already have a smart watch that is tracking your heart rate, you may be able to infer this condition by monitoring heart rate deviations from normal.