The most critical thing to look for is a pause in breathing for more than 10 seconds. But be careful: breathing can be an illusion. When Mr. Lawn Mower Mouth suddenly falls silent, you may look over and see his belly moving up and down and assume he is breathing. But he actually might not be. That movement might just be his diaphragm trying to pull air down. His gut can be moving even if he's not breathing air in.
What you want to do is listen for changes in his breathing. Snoring is actually a good thing (never thought you'd hear that, huh?). It signals that some air is actually moving in and out.
If his regular snoring suddenly stops, it can be a warning sign that no air is moving in and out. If there's no sound for 10 or more seconds, and there isn't a "whoosh" of air out of the nose or mouth, it's an indication he isn't breathing.
Find out more about this book:YOU: The Owner's Manual, Updated and Expanded Edition: An Insider's Guide to the Body that Will Make You Healthier and Younger