control after clicking add to cart button

I'm running v1.5.1.3 at localhost.
I would like to check a value from the database just after the user clicks add to cart button. More specifically, I'll check the manufacturer_id from the database and if the id is same with the one that is already in the cart then I'll add that product. Else I want to display an error message.
By the way I have to check from the "current cart" because customer_cart value(from database) does not gets updated right-away (I guess it's a bug!) I have to make another click(o rF5) to actually see the item in customer_cart database.

Thanks