However, I see nothing visually apparent in the displays to suggest they're outgassed either -- but I haven't had much luck sourcing information or photos on what one should look for.
If the display is outgassed, you usually see little white / gray dots scattered throughout the display. I found a photo that shows a typical example.
I did this when I got my Taxi. It's been a while so I don't remember exactly but you have to remove some resistors from the display board. This is because the jackpot display "steals" the segments from the lower display (that's why the lower display isn't alphanumeric!). If you compare the display board from whirlwind and taxi, you should see where there are a few missing resistors.