The only way to find out which deviant it is before she grows up is to research her genes. If you look at my foal here: http://beastkeeper.com/pet/301520
I researched her colour genes and it shows me that Cream (so the Pattern gene, not the Colour) is the one with the Deviation. So now I can go to the Skin Library (Help -> List of Skins), scroll down to the table with the "Normal Colour, Deviant Pattern" title and look for Cremello. This tells me that my foal will look like this when she grows up:
Sadly without knowing her genes, you won't be able to predict her looks.
EDIT:
Hmm, actually this is not entirely true.. We can look at her lineage too and take a few guesses. Neither of the parents have all colour genes researched, but given that your foal must have the Piebald gene to become an Overo, which it could have only gotten from the mother (father's dominant gene is Normal. If he had a Piebald in there, it would automatically be dominant over Normal). This tells us that the foal can't have double pattern gene. Therefore she must have the Chestnut gene double. So now we check in the SKin Library again, scroll down to Deviant Color, Normal Pattern, we see that your Chestnut Overo foal will look like this: