You can either use the "Enter" event or the "BeforeUpdate" event.

Using "Enter" can be a little problematic, as sometimes Access automatically
enters a field (for example, if it is the first on a subform) and triggers
this event when all you are doing is loading the form.

"BeforeUpdate" gives you the opportunity to Cancel before it actually posts
the value. It occers just as you try to post the changes.

Sub YourField_BeforeUpdate(Cancel as Boolean)
  If MsgBox("Did you check so-and-so?", vbYesNo) = vbNo then
       Cancel = True
       Me.YourField.Undo
  End If
End Sub

- Phil Freihofner