First, because the union could negotiate that as part of their initial agreement with the company: "don't use any means to entice or threaten people into not joining the union". If it's that common a tactic, that would presumably be a standard part of a union's playbook.

As for how they'd know it was going on: it only takes one person to "leak" that information to the union. And if the union and company are large enough that new people get hired regularly, they might occasionally find someone who fits the profile and send them through the hiring process specifically to find out how a new employee gets treated.