You can read out the WSDL to see what the webservice expects/returns.
If you mean to track how often your web-service is being used you could track how often your webservice user logs in and out in the log files. I believe you can also add a java action to print to the log and put that in your published web-service's microflow.
Edit: Try adjusting the loglevel on your server on DEBUG or TRACE. These might dump the entire webservice call to the log. Be very careful with this though as it might spam your logfiles if it's a busy server. Your only other option seems to be to run it locally and call that the same way you call it the production server. On a server running in Development/Test you can go to Advanced -> Set console log level -> Webservices and set the log level specifically for your webservices.
What you want is not yet possible (it will be in 2.5), however there is a workaround to get the full SOAP request and response of a call to your published web service. You need to call your web service using another Mendix application (this can be the same application if you like). By setting the log level of the WEBSERVICES module to DEBUG for the calling application, you can see the SOAP request and reponse of the web service call.
Of course, this still does not allow you to see a request that is sent by a third-party application. If you want that, you should try to get this information from the third-party application.