A healthy list doesn't always mean that it only brings you $$. That is an advanced step and the content you provide could have a great impact on your revenue from the list.
Each list should have some KPI's to be analyzed after:
1. Bounce rate
2. Open rate
4. ROI (but this can be influenced by the action taken by competitors and by your own content)
As i said before, return is just a result of two actions: getting the right people in the list and providing the right content.
When in doubt about the health of a certain list i would try to first segment it by the information provided by users and also validate every single email address to have a clear input on the right size of your list.
For the email validation system i would suggest trying ZeroBounce.
For the email segmentation part i would suggest using MailChimp (it seemed a little basic at first but you can do wonders with it)