RadInput does not readily offer binding to web services for value checks. However, you can implement this functionality both on the client and on the server.
On the server, you can have RadTextBox postback on losing focus by setting its AutoPostBack="true" property. The server-side event that is fired is TextChanged. There, you can check your changed value and display a message. More information on the TextChanged event of RadInput you can find here
If you want to perform name checking on the client, you can use RadInput's client-side API, particularly its OnKeyPress client-side event. More information on the topic you can find here
All the best,
the Telerik team