I have the same question too @williamsyee
Reflection and emotions are key here. The quote says "happiness is getting what you want".
I reflect on my journey, every big decision made, every road taken and my feelings that comes with them.

1. Am I happy with my current situation?
2. Am I happy with what I have right now?
3. Am I moving towards my dreams in my current situation, job, etc.

Happiness is an indicator whether your ladder is leaning on the right wall or if you're using the right ladder 😁