stopping someone from tabbing past a certain text field

This is probably an easy one.  I have not done any Cocoa dev work in a year and any real work in about 2 years.  

I have a panel with some text fields.  2 of the fields need to be filled out and I would like to validate them to have something in them.  I can do that using the delegate method

control:textShouldEndEditing:

but that only gets called if they start to edit it by typing something.   Tabbing into and then out of does not trigger the edit so this (and other edit related delegate methods) do not get called.

What is the best way to be able to check as someone tabs through that they have edited it.

I could of course wait until they hit the accept button but I would like to provide feedback as they work their way through the form.

Thanks!
Chad