Almost all cables are color-coded to help you remember the difference: the black connector for master, the grey one for slave, and the blue one for the motherboard. Some cables actually have the words "master" printed by the end connector, and "slave" by the middle one.

According to one of the articles I linked earlier, the reason for this marking scheme is to prevent someone from dangling that end connector "loose" in a single-drive configuration (which I *do* acknowledge and understand). Not because the device order is critical in a two-drive configuration.