https://github.com/streaak/keyhacks
Has a list of many ways to test an API Key for validity.

Porting these to KeyScope would be useful.

• ABTasty API Key
• Algolia API key
• Amplitude API Keys
• Asana Access token
• AWS Access Key ID and Secret
• Azure Application Insights APP ID and API Key
• Bazaarvoice Passkey
• Bing Maps API Key
• Bit.ly Access token
• Branch.io Key and Secret
• BrowserStack Access Key
• Buildkite Access token
• ButterCMS API Key
• Calendly API Key
• CircleCI Access Token
• Cypress record key
• DataDog API key
• Delighted API key
• Deviant Art Access Token
• Deviant Art Secret
• Dropbox API
• Facebook Access Token
• Facebook AppSecret
• Firebase
• Firebase Cloud Messaging (FCM)
• FreshDesk API Key
• Github client id and client secret
• GitHub private SSH key
• Github Token
• Gitlab personal access token
• GitLab runner registration token
• Google Cloud Service Account credentials
• Google Maps API key
• Google Recaptcha key
• Help Scout OAUTH
• Heroku API key
• HubSpot API key
• Infura API key
• Instagram Access Token
• Instagram Basic Display API
• Instagram Graph API
• Ipstack API Key
• Iterable API Key
• JumpCloud API Key
• Keen.io API Key
• LinkedIn OAUTH
• Lokalise API Key
• Loqate API Key
• MailChimp API Key
• MailGun Private Key
• Mapbox API key
• Microsoft Azure Tenant
• Microsoft Shared Access Signatures (SAS)
• New Relic Personal API Key (NerdGraph)
• New Relic REST API
• NPM token
• OpsGenie API Key
• Pagerduty API token
• Paypal client id and secret key
• Pendo Integration Key
• PivotalTracker API Token
• Razorpay API key and secret key
• Salesforce API key
• SauceLabs Username and access Key
• SendGrid API Token
• Shodan.io
• Slack API token
• Slack Webhook
• Sonarcloud
• Spotify Access Token
• Square
• Stripe Live Token
• Travis CI API token
• Twilio Account_sid and Auth token
• Twitter API Secret
• Twitter Bearer token
• Visual Studio App Center API Token
• WakaTime API Key
• WeGlot Api Key
• WPEngine API Key
• YouTube API Key
• Zapier Webhook Token
• Zendesk Access token
• Zendesk API key