Idling, on a level surface. If the sight glass is totally full, you probably won't be able to tell if it's empty or full; you might look at it first with the engine NOT running (it will be a bit lower) to see if you can see the fluid line in the sight glass. Then start and look again. Better yet, stand back there looking while someone else starts the engine.
The reason the transmission has a big sight glass, and not a dip stick is the oil level isn't real critical. If it's anywhere in, or close to the sight glass, it's going to work fine.
It can be hard to tell where you're at, especially if you fill it above the sight glass. So, take your time with the last gallon.