Find your local/regional arcade group on facebook and join it. Someone will always mention any auction well before it comes along.
However the auction scene isn't a fraction of what it used to be. They have always been largely fueled by operators getting rid of obsolete equipment, and warehouses being completely liquidated. These days there aren't a whole lot of traditional operators left and when they do get rid of something it is usually the gutted shell to some 15 year old driving game .Everybody just runs everything forever these days, since new games are so few and so expensive, that traditional model of replacing the equipment every few years has completely broken down. Meanwhile most of those old warehouses have already been cleared out. While large warehouses of old games still exist, they are largely in the hands of those who make their living directly from those old games (and not traditional operators who are bad at math).
When I started in the hobby there would be 2-3 regular consignment auctions locally a year and at least one non-recurring one (location closing, etc). The last time there was a consignment auction within 100 miles was in 2012 and there hasn't been a location closing auction locally since 2013.
Also, I must mention that most of the recurring auctions have now been ruined by online bidding being added.