Dave,
open the parent form in design view.
Click only once on the subform - you should see black handles around the
edge of it - the same as if you clicked a textbox, combo, image control etc.
Then click on it again, the black handles disappear.

The black handles are on the edge of the subform control.
The second click takes you into the subform itself.

Click on the main form then click once on the subform control and see the
black handles.
Look on the Other tab and find the name of the subform control.
It might be called Child followed by a number, or it may have a different
name.
Have a look on the property dialog on the Data tab, you should see Source
Object at the top of the list.
The Source Object gives you the name of the form that is currently in the
subform control.

You can swap different subforms into the same subform control ( more
advanced stuff)

Jeanette Cunningham