Google recently introduced its Google Ads API Doctor. Google built the tool as a response to continued developer feedback regarding the difficulty involved in configuring client libraries and provisioning OAuth2 credentials. In short, the tool verifies whether OAuth2 credentials are configured correctly and guides developers through fixes to OAuth2 problems.
Two common problems version one was designed to address are the invalid refresh token and permission denied. When an invalid refresh token is identified, the tool will guide the developer through obtaining a valid token, backing up a configuration file, and writing a new value to activate the file. When permission denied is identified, the tool identifies which of the many reasons this has occurred and so the problem can be remedied.