There are 2 test modes that can be used when working with this part of the API.
Test Mode 1
If the parameter 'test' is set to 'true' by itself, you get a simple welcome message. This can be used to verify that you are able to successfully connect to the API.
Test Mode 2
Adding the ICAO Station ID parameter to the Test parameter will return a web page that tells you the current version of the API you have connected to, all the possible parameters with their current values, the raw URL of the current observations, and the XML sent by NOAA's National Weather Service for the current observations. Information on the ICAO Station ID parameter can be found here.