Currently there is no possibility to access all RadControls on the page and apply some method to all of them. Each particular control in our suit is specific and provides different methods and properties on the client, so the same method for particular action could not be applied to all of them.
I would suggest you in your case in order to validate each control on the page, to validate the whole Page itself. You could call Page_ClientValidate() function and check to see if the validations(Custom, RequireField) are complete in the page.
All the best,
the Telerik team