Learning the API
It is necessary to learn the API to gain more insight into how it is structured. This includes reading developer docs, making hundreds of requests with different request methods to a single endpoint and observing how it responds, and learning roles (user roles) if any that may be implemented, and understanding scopes related to the access token.
Developer documentation
A developer's documentation gives a great insight into any API. You can learn about API endpoints which are already available publicly for use. One can understand structure, data-types, permissions, and types of request methods, which are accepted by the endpoint.
As an example shown below, Facebook's Graph API documentation gives a great understanding about any endpoint. We are looking at documentation of a user endpoint. Developer docs give you an idea about which request methods are accepted by the endpoint and what type of data needs to be sent at that endpoint.