jQuery Validation - manually set field invalid

jQuery Validation - manually set field invalid

I have a form using jquery Validation that was done by another developer. Rather than using the built in remote option, he put in his own onblur on one specific field in the form that shows a box with a special error. Is there a way to somehow add a class or something to the input which tells the validate() to fail? I've googled a bit and not seen it.

Suggestions?