You might do well to ask your customer if they have the Programmer's Reference Manual for the printer. Some manufacturers ship one with every box; others sell them separately or make them available on their web site. In order to find this in the latter case, you'll need the exact make and model of the printer on which the labels will be printed.

Of course, if the manufacturer regards this as a trade secret or something, you'll probably be going back to the reverse-engineering approach you've outlined above.