If the field coil is out of the engine, just get a DC voltage source such as a trickle battery charger, or even a 6 volt lantern battery. Clip the negative lead to one of the mounting bosses and the positive lead to the contact "button" at the top of the field coil. Have a small file or other steel piece ready, and after turning on the power pass it over each pole face. If all windings are good, the steel will be attracted magnetically to each of the 16 faces.
However, it's a much better idea to replace any original field coil that has
ever been in an engine with a rewound unit. The amount of dirt and mettalic
debris that that used field coil contains is surprising, and you sure don't want
to put it into your engine. Another factor is the coating. A field coil must be
coated with an oil proof and heat resistant insulating coating, and the original
is suspect even if it looks perfect because of the many modern oil additives and
detergents that can attack it. An original field coil cannot be coated
satisfactorily because it is next to impossible to get all that old oil out.
Even if you can remove all you can find with solvent and detergent, when you've
got your new coating applied and you attempt to bake it, that oil will come
oozing out all over and will take off the new coating with it. Save time,
aggravation, and money: replace it with a rewound, recoated unit.