Cart check on checkout. Which function can trigger?

[Adwans]

Hello,
I m going to check something in cart object (manufacturer, quantity /total sum per manufacturer)  and this should be triggered by checkout  button. Just before creation of an order.
Which VM trigger function can do it? I should work like "function checkPurchaseValue()"  in helpers/cart.php  - stopping transaction with VmInfo or VmWarn displayed.
I searched whole forum , some similar topics but I need PROPER trigger   Hope that lots of your plugins use this function .
Thanks in advance.

[Adwans]

Thanks , I am closing my post now. This problem (checking content of a cart) demands another approach, not only plugin-based  but with some JS as well.