How To Verify A Google Maps Api Key In Html Javascript?

I have a rather standard Google Maps page, using my own API key. The page only does some marker additions by JS and presenting it in a text-box. If the key is invalid or expired or

Solution 1:

Add this:

function gm_authFailure() { alert('yo'); };

In the global scope. Per this:

https://developers.google.com/maps/documentation/javascript/events#auth-errors