On pre 1998 Fords, the master key is a solid red key. Slave keys are black with a red chip.

One post 1998 Fords, the first 2 keys programmed into the system are the masters (you turn one in the ignition, remove and do the same with the 2nd to open the system for programming), regardless of remote key or not. Any key programmed after that is a slave key. You can have 4 remote keys and a maximum of 6 or 8 keys total on the later systems.