How well I sleep. Am I worried or confident? What keeps me up at night? This may not work in your 20s but the older you get the less you can bullshit yourself, especially not at 3 o'clock in the morning. Though it is certainly possible and quite likely to have irrational fears, the quality of the fear doesn't matter; my metric is to have as much inner peace as possible.
Success in life for me is how much free time I have to work on things I want.