If he actually cares about not hiring illegals then he could enforce contracts with those subcontractors that included penalties for using illegal foreign labor.
Has he done that? Or has he just enjoyed extra profits by taking the lowest bidder using illegal labor?
By law Trump can’t go above and beyond to get proof that his contractors or subcontractors aren’t hiring illegal labor—that is considered discriminatory.
All that can be done is to ask for the workers to provide the requisite papers—and then they must be accepted, however fraudulent the papers must be.