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.