I sought in Internet how to determine from Google response whether an access token is expired. I found no definitive answer.
So, finally I created an access token and waited for a hour to see the Google response when I try to use this token.
The Google response (with a partial list of HTTP headers) is below:
HTTP/1.1 401 Unauthorized WWW-Authenticate: Bearer realm="https://accounts.google.com/", error=invalid_token Content-Type: application/json; charset=UTF-8 Username/Password Authentication Failed.
A weird thing is that Google says that it is application/json but the response body is not a JSON.
So the only way is to check whether the response HTTP status is 401.