The main point for me is that you don't only need a good definition of left (/right), but you also need to keep your audience in mind.

If you ask me "what type of person doesn't know what 'left' means", my answer would be "either a child or a foreigner who just started learning the language". For that audience, even saying "the side where your heart is" (like some other comment suggests) would require knowing what "heart" means, which might not be a good assumption for this specific audience.

So how would you propose to explain which side is which?