It is possible to test an API directly from the Developer Portal. When looking at the details of an API you will see a table of the operations contained in the API.
The Developer Portal will show you all the methods available, GET, POST, DELETE, etc, and what are the paths to use.
If you click on the resource you will see more information about it, what parameters it might take, what it returns, what possible return codes it might use and what they mean. There is also a 'Try' button which enables you to try the resource out direct from the Developer Portal.
If you have any doubt or any feedback, please let us know.