This is the ideal way, but if the plugs are really difficult to access I'd use a sight glass and slowly add fluid until I can just barely see it. A slight overage (just a trace ammount within the glass) shouldn't hurt anything.
Keep in mind the fluid might not drain away easily from the glass after use and give you a false reading.
If I did a fluid change, I'd take the glass out to save time on filling. It is very easy to add too much.
If I thought I was getting water or another fluid in the tranny (or other reservoir), I'd totally use a sight glass. That would be a slick way to quickly check daily.