As you already observed, you can use RadInputManager
to validate textboxes toconfirm they have values or in their values correspond to particular regular expression etc. However, I am afraid we do not have such extender for RadComboBox. What you could try in order to display the error message in the combobox, you can try adding an item to the combobox on the client when validation is fired.
For more information on RadComboBox client-side API, you can review this help topic
the Telerik team