At the moment, theoretically, they treat "being pregnant" plus any of the other symptoms, or a maternal report of general discomfort, as a reason to investigate further. Increased bp or bad bloodwork or a headache or decreased fetal growth or decreased fetal movement or brisk reflexes or unusual swelling usually trigger some sort of followup.
Soon they will be able to test bloodwork for the levels of two proteins and the rate at which they are changing and be certain that you have PE -- or even that you will develop it before 34 weeks gestation. But right now that test is still in development.
High protein alone isn't a reason to deliver because the group of women (and babies) with crazy protein readings doesn't actually do worse than the group with mildly elevated readings. Delivery is usually timed by things like escalating unresponsive blood pressure, or the sorts of reflexes that make them worry about seizure, or a fetus who's becoming unresponsive, because in those groups there's a clear benefit to delivery. It turns out that you can have crazy proteinuria and not really be threatened by it, though, plus it reverses after delivery.