Not clear if you mean if fields have changed on the modal form or the form that opened it. If the modal form and it is bound I think you would have to set a variable in the form AfterUpdate event because once saved, you can't use the .Dirty property of the form. If you want to know which control(s) specifically, then I'd try looping over the controls by type and comparing their .Value to the .OldValue to see which were altered. If not bound, one way would be to concatenate a string variable of control names as they are updated.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.