Updated: Mar 10, 2021
How do you know? Well, pay attention to them!
It seems simple but the more familiar you are with your dogs mannerisms - the more easily you can tell when they are unhappy. Make sure they have adequate exercise and play time. Ensure that clean fresh water is available at all times. Feed on a regular schedule and stick to that schedule as well as you can. Dogs like structure so providing this for them is essential to their happiness.