If you are using AEM 6.0, there are some out of box client validations you can perform on the AEM adaptive forms fields.
You might have to write client site validation scripts on dialog fields depending on your requirement.
You can do a combination of client side and server side validation, details documented in the following link
http://www.citytechinc.com/us/en/blog/2012/04/complex_cq5_dialog_validation.html
Or you can have just client side validations using javascript, some examples described in following link
http://sonycharan.blogspot.com/2014/03/client-side-dialog-validation-in-cq5.html
AEM commons package provides some kind of client side validations too, you can find more about at
http://adobe-consulting-services.github.io/acs-aem-commons/features/widgets.html