Thank you for going to test this yourself.
Be aware, though, that there are some sites that offer fake bookings and if they are honest they say so on the site, eg
https://keyflight.io/fake
May I suggest, therefore, that you use the site mentioned in this topic, onwardticket.com, for your test and have your friends in the airline industry run it through the motions that check-in staff use.
Somebody mentioned PNR (Passenger Name Record) but looking at my wife's recent flight tickets they show no PNR. Someone mentioned that the passenger's surname and the 8-digit alphanumeric booking reference is typed into a form on a webpage that lets anyone, not just check-in staff, verify any ticket of any IATA airline and any passenger. I once had the URL but forgot it. amadeus.com or something like that, or perhaps that one is outdated and something new is used now? Then, of course, the ticket can also be looked up on the site of the airline that issued it, but the passenger's own login is probably required for that.