To test Web API query we can make use of Advanced Rest Client chrome extensions.
Log in to CRM Online and from Settings > Customizations > Developer Resources
Get the Service Root URL
- Launch Advanced Rest Client in Chrome
- Put the URL copied.
- Here we would also need to set headers, content type and Authorization.
- To get the Access token
- Log in to https://xrm.tools/AccessToken with your CRM Credentials.
- Select CRM Organization
- Copy the Access Token and use it for Authorization Header in Advanced Rest Client
Click on Send and to get the response
Update the query and click on SEND
More details here
Hope it helps.