back in the day, we could get Monobloc float bowl covers with a glass (more probably plastic!) window in it so that the fuel level could be seen.
They were useful for setting-up carbs, though some people used to run with them all the time (and with finned float bowl extensions
Are they still available ?? (must do a Google Search) ??
My avatar shows the late Len Rich in 1970 with the bike I now have - a 1958 Ariel VH