In general, you can check whether a Google-issued OAuth 2 access token is valid by calling https://www.google.com/accounts/AuthSubTokenInfo?bearer_token=OAUTH_2_TOKEN
and see whether you get back a HTTP 200 or 40x response.
What makes more sense is to do a "cheap" authenticated YouTube API call, like for https://gdata.youtube.com/feeds/api/users/default?v=2
, and see if you get back a valid response or an error.