Detect which subform is active

In Access 2003, I have a form Main that is a holder for several subforms, including Action and Task.  (They aren't linked to the MAIN form, it's just a shell/holder.  I have these subforms on fixed positions on the Main form instead of floating for various reasons that are *currently* more important that the feature I'm asking about here).

I would like to be able to set a global string variable TaskOrAction to either "Task" or "Action" when that respective subform is activated.

I need to find an event that I can use to detect that the subform is activated, or, more specifically, if ANY control on that subform is used, so I can set the value of TaskOrAction (which I use later to inform a query of whether to look for a key value in ...Task or Action).

I tried a few things that didn't work:
1)  According to MS, "Activate" does not work for subforms,

2)  According to this, http://www.access-programmers.co.uk/forums/showthread.php?t=191507
GotFocus does not work unless you use the record selector (though I did not see that happen either, when I tried GotFocus

3)  Enter only works for specific controls, and I need to set the variable if any control on the subform is used, or if the subform record selector is keyed.

I built a little test bed, attached.

Any suggestions on this would be appreciated.

Thanks!

Test.mdb