You can use the isAjaxRequest property of the RadAjaxManager instance to identify when an Ajax call is in progress. If you are really curious you can inspect the Request.Forms collections and see how Telerik makes use of the underlying PostBack variables.
For a simple example I worked up, here is what was returned from an Ajax call to the server:
Request.Form["ScriptManager1"] "AjaxMgrSU|AjaxMgr" // Telerik house keeping
Request.Form["__EVENTTARGET"] "AjaxMgr" // ClientID of the RadAjaxManager
Request.Form["__EVENTARGUMENT"] "firstName=Carl&lastName=Taylor" // My stuff, just a simple string to their architecture
Since I am creating/updating some custom stuff on the client. I am trying to figure out whether I go with Telerik or my own stuff for this particular page.
In closing, please keep in mind that I am no expert, so please verify what I have told you because some of it could be incorrect.