If you are building an installed application that uses OAuth, you may want to display the Email address of the user's Google Account. To do this, you will need to modify the scope(s) you request via OAuth to include the following Email Display Scope:
https://www.googleapis.com/auth/userinfo.email
Make sure you separate each scope you request with a space character in the OAuthGetRequestToken step.
Once you have a valid OAuth token, you can use it to make API calls to the Email Display API endpoint:
https://www.googleapis.com/userinfo/email
If the token is not valid, a 401 error will be returned. If the token is valid, then the user's Email address will be returned. The API will also return a boolean value to indicate whether Google has verified that the user owns that Email address. However most installed applications will ignore that value.
The API endpoint by default will return the result in a simple URL encoded format, i.e.
email=fred.example@gmail.com&is_verified=true
If you include the URL parameter alt=json then the format will look like
{"data":{"email":"fred.example@gmail.com","is_verified":true}}
If you include the URL parameter alt=xml then the format will look like
<?xml version="1.0" encoding="UTF-8"?><email isverified="true">fred.example@gmail.com</email>