Were I to venture a guess I would suppose a child like attempt with white out and a copier or being snitched out by colleagues after flapping his gums about it. Verifying every record in the NHL seems a daunting expensive task. Checking records for every state employee in Washington would be near impossible. After canceling out in 2020, after I got vaxxed I started doing my old man routine visits to specialists and dentist and optometrists. Not one office would even look at my vax card or make a copy. When I asked why a lady in one of the offices said their management would not take on the liability to verify if a card was real or fake. They were instructed to NEVER look at a card. That pretty much convinced me there were fakes aplenty. Blood tests would be accurate but would require a medical records release. Vaxes may be mandated but a mandate will not get around HIPPO(sp) privacy laws. JMHOSan Jose Sharks' Evander Kane suspended 21 games for violation of COVID-19 protocols - CBSSports.com
It doesn't say how the NHL found out. But there are several methods. It can be as simple as checking to see if the lot number was distributed around the date on the card and in the geography in question.