Acc 2000: Subforms - Test Calling form for Data first

Is there a way to check if a main form is dirty (other than locking or
disenabling the subform), BEFORE turning over focus or control to a subform,
so "damage control" can be done in advance?

My main form has some default values and one optional field. If the user
chooses to accept the defaults and enter nothing in the optional field, the
main form will never get dirty. Thus, behind the scenes, I'd have to set
the values of the fields with defaults, and save the record, beforehand.

Thanks in advance for any help you can offer.

Bari