The only test worth doing is using your ears. The good ones are very good, and if they perform ok - then use it. I've compared the legit SM58s I have going back a VERY long time, and the weights and internals do change over the years, so a catagoric yes in almost impossible. The others jump out as just poor sounding, poor looking or poor everything.
I'm familiar enough with them to simply be happy or unhappy with the sound, and if you had two, I guess you probably can do the same. If you paid peanuts and it sounds good - it is good. Nobody pays a lot for a second hand 57 - this one looks very new and shiny, which is a bit odd. Second hand 57s rarely look new.