Access 2000 setfocus to another form

I have a subform used by two different main forms. On the exit event of one of the subform’s fields I want to setfocus on a field on the open main form. How can VBA determine which of two main forms is open so that I can set focus to a field on whichever main form is open? I was thinking of the IsLoaded property but don’t have a good example to go by.