Labels are a good starting point, but, like wire color, should always be verified. All too many people “make do” when they’re rushed, leaving the next guy with potential problems.
If you don’t know how to tell which is which, most Home Depots have good classes on how to install light switches that can be very helpful in getting started. They won’t necessarily cover networked switches (it depends where you live), but they can keep you from burning down your house and give you enough knowledge to understand the next set of questions to ask.
For the Master switch, this FAQ may help:
The traveler is used for communucation between the master and the aux.
There were actually two “travelers” (where “traveler” is a wire that goes between the two switches).
I wire-bolted one of them in the “dumb switch” gang box to the line (which I identified with a non-contact voltage meter). Then I connected the other end of that same traveler to “line” in the other gang. I connected the remaining traveler to traveler in each gang, load to load in the primary switch gang box. Connected neutrals. Everything works.
So - important lessons:
With the power off, Use a spare length of wire, a wire bolt, and a meter in continuity mode to identify the travelers.
With the power on, use a non-contact voltage meter to identify the line and load.