In commercial or industrial electrical work how would you take a voltage reading?
This is a middle school science level question. No offense, but
if you don't know how to do this you should not be doing any
electrical work, let alone commercial/industrial. If you are a
homeowner, please buy a book. If you are a professional, please
take a class and get licenced. Get a voltmeter. Set it on AC volts.
Use the two leads to measure the voltage across the two points you
are interested in.