What's a good way to measure happiness?

Happiness cannot really be quantified. Best thing you can do is check in with your mood at the end of each day and if you usually hit the KPI's (key performance indicator) for the other two big life arenas (health and wealth) then you will be overall satisfied and happier at the end of the day.

You can also try this mood meter app here: https://moodmeterapp.com/