Cart Variant and Add to Cart Button

[chicago]

Hi,

Using Joomla v 2.5.11 and Virtuemart v2.0.20b

I'm creating a Shoe Store for a customer and every product has a Cart Variant 'Size'; (some items may have an additional Cart Variant).
Is there any way that I can show an error message "please select option" if the Add to Cart button is pressed without selecting an option from the Cart Variant?

Any help would be appreciated