For WatchOS, you would start by running the Sleep App[0] and notice that there is an issue with your sleep stages.
Next you might move onto an app, like SnoreLab[1] where you record the audio of your sleep each night and can listen back to where you are having sleep interruptions. At that point, you would likely hear your breathing process.
Moving onto the Withings Sleep Analyzer[3] (as shown in the article) will fill out your data but by step 2, you will should probably already be setting up an appointment with your doctor for sleep apnea.