In hindsight, can you see when your long term relationship began to go downhill?

This question was inspired by another where people stated they were totally blindsided by their SO leaving or admitting they were unhappy. Most people stated they did not have a clue. Really? Were there truly no signs?
They say hindsight is 20–20. Can you look back 6 months, a year or more, before the “surprise” and see any signs it was coming?
For example: reduced or no desire for intimacy, SO starting an exercise program without you, new clothes, more travel, more arguments, etc. What do you see in hindsight?

Had you known your SO was that unhappy, do you think you could have changed something to influence the outcome? Would you want to? Will you do things differently with your next SO?