With the changes to NFPA 70E's 120.5 Process for Establishing and Verifying an Electrically Safe Work Condition, through the last few editions, I'm glad they kept the first step
(1) Determine all possible sources of electrical supply to the specific equipment. Check applicable up-to-date drawings, diagrams, and identification tags.
But unfortunately not all drawings, diagrams, and identification tags are ever quite up-to-date. It is best to have a group discussion with all electricians involved before work begins. Allow time for individuals to review these documents and if possible get eyes on the equipment before the meeting. Also, if a customer or effected employee is knowledgeable on the system and present allow them to be a part of the discussion.