This is so simple you are going to say--WHY DIDN'T I THINK OF THAT? Having
done route service for years it was always hard to tell which side of the
playfield glass was up when you slipped the glass out for a quick repair.
Which side has the customer's finger prints and etc. The solution is
simple--just put a small sticker on the glass, on the bottom side (won't
stick to "beer seal") low enough that the lockdown bar covers it (no body
else will ever see it.) We used self-adhesive pool table spots, only
because we already had them in our pockets, but any sticker will work even
just a piece of tape. Then a quick glance tells what side was up.