Look at the request (ServletRequest) object properties and methods. In the "Guess My Number" example, the handler is defined as
Sub Handle(req As ServletRequest, resp As ServletResponse)
The req object contains the request (the client data).
req.InputStream would give you the data that the client sent you (in raw format)
req.ParameterMap would give you all the parameters that the client sent you (either through the URI (via GET request) and/or body (via POST request))
req.FullRequestURI would show you the URL used to access your page
req.GetMultiPartData will help you with request that were POSTed multipart
You can log any of these (and anything else) to see what is coming in