Those ”lights” are simulated, so if they’re being shown on the top of the VX820’s screen you can be sure contactless is enabled and apple pay will work. A Verifone VX820 with the contactless disabled shows no “lights” on the top of the screen at all or contactless logo, but rather only a message that says “insert or swipe card” and maybe the payment provider’s logo (though the latter isn’t very common in the US unlike in Mexico). I know that because all but one of the retailers using the VX820 in Mexico have contactless disabled and that’s what it looks like when the cashier unlocks the reader to process a card payment. Only at the one supermarket chain in Mexico where they have the NFC enabled the vx820s show the rectangular green “lights” at the top of the screen.
I've actually seen contactless not work despite having the four rectangles. The key in my experience is the leftmost one needing to be green and not just gray/clear, which I wasn't able to confirm.