There are two ways to check whether a story is currently behind the paywall and eligible to earn.
Option 1: From the story page
Open the story you want to check and look for the member-only story badge with the star icon at the top of the page. If the icon is visible, the story is behind the paywall. This works the same on both web and app.
Option 2: From your profile
All your published stories are visible on your profile. Paywalled stories are marked with a star icon near the publishing date. This also works the same on both web and app.