Hi Ankit,
I see that you have two concerns. Below you will find my answers to each concern.
1. Conditions in which an account will not be eligible to serve ads.
These conditions include when the account is a test account that cannot serve ads, or if billing has not been set up. In these cases, an account will not be able to serve ads.
2. How to know if an account's billing setup is set or not.
Unfortunately through the API you can only see whether they've created budgets for the account. There isn't a field that correlates to if the account's billing setup has been set or not. I can add a request for this feature to be added if you would like. In the meantime, please follow our
blog for updates on updates and new releases pertaining to this concern.