How do I know which pages should be deindexed?

You can check your Google Analytics account to identify pages that attract very little traffic from Google.

Typically, these are pages that Google does not particularly like.

If these pages also have little content, there is a high chance that they need to be deindexed.

On this page: https://support.google.com/analytics/answer/3306157?hl=fr you will find a custom report provided by Google to see which pages receive traffic from search engines.