I think that's your answer... Show them your existing front-end code, and ask them how they'd make it better. If they're experienced, then they should have some very concrete ideas. Then ask them "why" for their ideas. If they can't fully explain the "why" behind the "what" they're probably just regurgitating things they've read, but don't actually understand the tech.
Apart from this, do you have recommendations or ideas for the take home tests for frontend engineer interviews? In a previous hiring run, the test we used turned out to be too easy because all candidates were able to implement it successfully and offloaded work heavily to third-party libraries.
Instead, we spend more time discussing real-world-oriented stuff, like what the parent is describing.
Personally, I like to orient the discussion toward failures and recoveries if the candidate doesn't do it on their own. You learn so much about someone that way.