We have used lift consultants to come and in and inspect our lift system after a major issue.
They identified that the lift service company was not fixing all the items, and were trying to bill us for items that under their contract should really be there responsibility. We now engage the same firm once a year or so do a check and make sure we keep them on the straight and narrow.
We spoke with four companies that do this and settled on one we have been very happy with. Please private message me if you would like the details.