I had to do this one one occasion at a social function when a big guy went down with some kind of seizure. (Watch Luca Brasi getting garotted in 'The Godfather' if you want an idea of what I mean)
I listened and felt for breathing but in less than a minute it was the colour of his face and lips that told the story.
Anyway, this video is short, sweet and to the point.
In the FAQs it points out that even if someone is breathing you probably won't do any harm. so I guess the message is if in doubt, get on with it.
https://www.redcross.org.uk/first-ai...-not-breathing