cgi form handler question

[DJ_3000]

Hi,

im writing a form cgi handler that handles data inputted by a user.

I would like to know how do I output the name of the field that the user
forgot to input.

eg I have the fields:

name
email
address
age
salary


the user forgets to input his email and age
how would i write a for loop to output:

email
age

for name in form.keys():
        print name

would output the field names that the user did inpuit.. i want the opposite
of this ta!

Thanks!