Greetings,
As of now, the played tracks are marked with a checkmark "✓" in the icons tab in browser.
What I would like to suggest is that you to implement in some future updates is color coding the font (same way that Serato does for example). It's much easier that whole track name turns blue for example than looking for that small checkmark in the icons tab.
You already have color coding options, you could also implement it that way (since I don't use color coding for my playlists I would also be okay if you added option with some neutral color to mark the already played track in that set).
Hope you'll consider it as I don't think this would be that hard to implement.
Best regards!