You can use the built in ASP.NET validation with RadToolBar. The toolbar 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.
If you want only some of the controls on the toolbar to trigger validation, you can use the CausesValidation property of the RadToolBarButton or RadToolBarSplitButton objects.
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 toolbar 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 toolbar and tool buttons (both RadToolBarButton and RadToolBarSplitButton) also have a ValidationGroup property. The toolbar or button only causes validation by those validators whose ValidationGroup property matches the ValidationGroup property of the toolbar or button. (The reason the default behavior is for all validators to execute on post-back is because the default value of the ValidationGroup property on both RadToolBar and validators is an empty string.)