I assume that you are using RadDataPager in its Unbound Mode since you are pulling the data by hand instead of binding the pager to a source implementing the IPagedCollectionView interface which is the standard Silverlight interface for paging operations.
Just like the stock Silveright DataPager (both pagers are 100% identical in terms of API), RadDataPager provides two events -- PageIndexChanging (which is cancelable) and PageIndexChanged. You can use those two events to detect what the user is doing. You don't have to rely on UI elements such as the TextBox. Business logic and UI should not be mixed. The event arguments of those two events carry all available information, which in this case is the page index the user wants to go to. That is all in terms of API that RadDataPager offers.
You other option is to bind the pager to something that implements the IPagedCollectionView interface. It will play nicely with it and call its respective methods.
I hope this helps.
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