This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
To track/monitor which client has requested your webservice, you will need to trap and log the incoming request along with the client ip.
With this logged data you will be able to check the clients activity.
Further for non-repudiation, please see WS-Security.
subject: How to monitor that client is calling my webservice and get the times of client request?