How to tell which one of my plugins are paid or free?

Usually premium plugins that came with the purchase of a theme are updated via an external updater, not the wordpress built-in one. That could be one of the clues.

One obvious clue would be that the plugin has some kind of login/licence key system.

But when it comes to taking a look in the code, I’m afraid it’s not an option cause the paid features can be implemented differently on every plugin.

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