The search bar is a good start, but there's an even more precise way to do this. Take your existing tags and run them through the Google Keyword Planner for more ideas, and to see how many searches there are on each term every year.
Then take each phrase that gets goo search and Google it yourself (in quotes -- that's important to just get search results for that exact phrase) and see how many other pages in the Google-verse are using that phrase.
Finally, divide the annual search number by the number of existing pages to get a mathematical representation of how competitive that phrase is. Even if it gets lots of searches, if there are far more pages already using it it Google, you'll probably never be found on it.
This applies to Etsy pretty much as well as Google.
I explain this in more detail here:
https://cleverruthie.wordpress.com/2014/09/08/a-sweet-secret-to-seo-and-etsy-happiness/This method works incredibly well for me and the Etsians who are now using it too.
Good luck!