How do I test field coils in a starter?

[mcgarrett]

I have a Model B starter that will start the engine on my test stand, but seems to be sluggish compared to a Model A starter I've been using. I've had it apart a couple of times looking for a problem and found things very clean inside with brand new field coils and new brushes. The solder connections look sound to me as well. Should I be checking the field coils to confirm there are no hidden problems? What other tests could I make? The battery is NOT the problem - it whips my other Model A starters around fine. Ground strap is clean and tight.