Is there a way to tell from a ballast's specifications (voltage, wattage, how many lamps it handles, etc.) whether it is a T8 or T12 ballast? Or do I need to look for the manufacturer's part number to figure it out?
The stock sizes of 4' bulb are F40T12 and F32T8. T8s are relatively new.
Both make about the same light, the T8 is more efficient.
So when they talk about F40, without any further commentary, and particulary when the unit is older, they mean F40T12.