Form Validation in Admin

Hi all,

I'm using the form's clean() method to do validation for models in admin. The validation we need to do is to make sure there are no more than a certain number of items totally submitted. So I calculate the total number of items in the clean() method and check whether it is in the given range. But the exact total number is difficult ot get because I don't know whether the newly submitted form'data is a modified item that is already submitted or it's a newly added item. So is there any way in the clean() method to know whether the form's data is a newly added item or a modified one?

Thanks to all.