Ive a one year old pup, border collie/labrador mix. I can take a quick glance at his face and know exactly how he's feeling. Probably 80% of the time, its pure joy. Happy to run around, chew on toys, jump on the furniture and try and play (unrequitedly) with our cats. About another 10%, he's scared of other animals he can see out of the Windows of our townhouse and barks incessantly. The other 10%, I have no idea.
Eyes on a dog don't tell the whole story. Need to look at eyes, ears, back position, tail and legs to tell.
As I write this, my dog is laying against me, calm amd near sleep. Depending on perspective, I could see his face as seeming sad, but I know he's just sleepy. He's only awake because I am and I've not put him to bed yet. But his face says to me he's calm and content. His breathing is also slowing. His favorite chew toy is inches from his mouth and against my arm and he's content to let it stay there; that's how I know he's tired. If you were to just look at a photo of my dogs face right now, he might appear sad, but I know hes just calm and sleepy.