To test the Google API, first of all, we require an access key for authentication purposes, and to get that, we should have a Client ID and Secret. In case you do not have access to those, you can follow these steps to get access to that information:
Now you can use Postman or other API testing tools to perform further testing. Once we get the ID and secret you can get the access key and using this key you can access the google API. Whichever google application you wanted to test you would need the API endpoints. Suppose you are testing Gmail API, then you can get the endpoints from the Google documentation mentioned below.
Google Document: https://developers.google.com/gmail/api/reference/rest/v1/users.messages/get
You can use the API method as per the requirement like GET, POST, PUT, PATCH, and DELETE.