Through daily information on volume going over the border. If you know what you're dealing with, then you behave accordingly.
And again, it's not as if mills have huge capacity, that they surge out on a daily basis. The infrastructure of softwood is a very well-established balance right now on logistics, railcar availability, trucking capacity, and so forth. Unless you have a mill right on the U.S. border, most of what you're dealing with is an even flow of product out of your mill.