When starting a project, I always ask for a list of the APIs and sample responses and requests including auth requirements, parameters and data types. Preferably there is a document that details all of the ones required but sometimes this is provided in a wiki or back of a napkin depending on how far along the backend design is.
Once I have that documentation, I typically want to test the calls to make sure that the documentation matches the actual API. There are a bunch of tools out there that can help you test the apis, and you can google them to find a good one for your environment. I like to use the Rest Console, a Chrome plugin. It allows me to do pretty much any time of call via Post, GET, Put or Delete, allows me to set custom headers and displays the server response.
Anytime that I have an issue with an API I plug values into the console to debug what's going on. It's faster and easier than debugging it via Eclipse.
