You can use the built-in ASP.NET validation with RadPanelBar. The panel bar can trigger validation of other controls on the form when it performs a post-back to the server. Simply set the CausesValidation property to True.
By default, all validators on the Web page must be successful before the postback can occur. You can limit the controls that must be validated when the panel bar performs a post-back, while still allowing other controls on the Web page to be validated, by using Validation Groups. The validator controls have a ValidationGroup property. The panel bar also has a ValidationGroup property. The panel bar only causes validation by those validators whose ValidationGroup property matches the ValidationGroup property of the panel bar. The reason that the default behavior is for all validators to execute on post-back is because the default value of the ValidationGroup property on both RadPanelBar and validators is an empty string.