Bratton's (as well as other suppliers) sell a little device that temporarily plugs into the top of the distributor. By running or cranking the engine it tells you with led lights if the polarity of the ignition is correct or reversed. The part number is 17140, cost $25.50.
They have been around for years, I bought one about 20 years ago when they were about $15. I took it to a club event when I first got it and found about half the cars had their ignitions reversed and the owners were not aware of it.
Markings on coils and colors of wires will not always tell you if you have the ignition polarity correct.
I carry this little device with me in my Model A and when ever working on someone's car I make a quick check of the ignition polarity. If it is incorrect the easy fix is to reverse the two primary wires on the coil, regardless of what color they are.
There is also the old pencil trick that has been around forever that auto shop teachers like to demonstrate, then hand the pencil to the class clown and have him do it.
Tom Endy