[jQuery] jquery.validate just checking the changed values

Dear all,

I want to use the great jquery.validate plugin.
I have just one question. There are fields which i have to check via
ajax calls. I just want to check them if they were changed.
I thought about one option that i bind the valid method to the
onchange event.
I would prefer to call the validations on submit.
Is there a way to do this ?

Thanks in advance,
David