The first symptom that you will notice when you are running into problems is that the client would not behave the way you want it to behave. As an example, there would be no output, or the wrong output.
Since the outcome of running a REST client depends on the request that you send over the wire and the response that you receive over the wire, one of the first things is to capture the messages and verify that those are in the correct expected format.
REST Services and clients interact using messages, usually in pairs of request and response. So if there are problems, they are caused by errors in the messages being exchanged.
Sometimes the user only has control over a REST client and does not have access to the implementation details of the service. Sometimes the user will implement the REST service for others to consume the service. Sometimes the Web browser can act as a client. Sometimes a PHP application on a server can act as a REST client. Irrespective of where the client...