The PS/2 protocol uses one line as data and the other as a clock. The USB protocol uses differential signals which means you would expect one line to be the inverse of the other. The nature of the two protocols and what you'd expect to see on the two lines is very different. You should be able to detect them by just looking at those two lines and detecting what's coming in. If you see the two input lines moving together but inversely, it's most likely USB. If the two lines are moving independently and not necessarily inversely, it's probably PS2.