How to know when you have a good 6 volt neg ground coil and how to test un marked coils when you don’t know if they are 6 or 12 volts and positive or negative ground?

Also do coils get week ? How can I know which coil will give me the best spark? You order a new coil from a parts store, “hey I have a 41 Buick, need a good strong coil.” He throws a box at me, “here take this one.”  How do I test this new coil under running conditions, before i spend 100 $ having it installed?

Yesterday a bunch of us car guys were attracted to a beautiful 1934 Packard.  The owner was standing by answering questions about his car then he stated that the only thing not working was his fuel gauge, a very frequent old car problem.
The responses started, did you check this and that, the owner said I’m not a mechanic, then the next reply was something like “you should bring it to Joe’s garage he fixed Mikes last week.” Any hints who he should talk to in the Hampton NH area?