Thank you for writing.
You can use the SelectedPageChanging or SelectedPageChanged events for your scenario. The changing event fires before the actual change of the pages and you can get the page the user is trying to navigate to from the event arguments. You can also cancel this event and prevent the page change all together. The changed event fires directly after the change and you can use the SelectedPage property to get the page the user has navigated to.
I hope this will help. If you need further assistance, I would be glad to help.
the Telerik team
Q2’11 SP1 of RadControls for WinForms is available for download (see what's new); also available is the Q3'11 Roadmap for Telerik Windows Forms controls.