Originally Posted by Kacee

Same way it's always been enforced. Primarily by dogs. You are also under observation during the entire entry process. And could theoretically be flagged by the CBP officer who takes your GE receipt, though the whole point of GE is that you're literally a "trusted traveler" and thus an unlikely candidate for secondary inspection.

I believe SFO has realized that having a separate officer who simply collects customs forms adds absolutely nothing substantive to the process.