Ensuring that fields correspond

[Daniel]

I have a site field and a department field on a form. The
dept field is from tDept and one of the fields corresponds
to the site at which the dept is located. The problem is
that if the user changes the site on the form but does not
change the dept, there is a possibility that the
department will not correspond. How can I remedy this?
Is there some way to check it on the unload event? Should
I reset the dept field to blank if the site field is
changed?

Thanks

Dan