How to validate on user event?

I need to validate on a custom column field on item fulfillment. Unfortunately, this is a static item list and the validation support is weak. First, you can't tell which row caused a field validation even to occur. But I also can't use it because I have a complicated validation rule where I need to get to some item fields that are not available on that form. So, I think I need to do the validation on a before submit user event. So, my question is how do you signal that validate failed? Do I just throw an exception? Return