If you create the standard IValidate class for the element you want you should be able to add custom validation which will be triggered when trying to publish. As per standard with IValidate you can raise these as errors, warnings or information. http://www.jondjones.com/learn-episerver-cms/episerver-developers-guide/common-web-page-component-tutorials/how-to-return-different-validation-warnings-to-content-editors-when-publishing-content-in-episerver