I second Mike, we miss more information.
Usually, it would not matter. For example, if you plug in an external drive or thumb drive via USB, it gets a drive letter by which you address it and that should be known to you. If it's another device, like a keyboard or mouse, that simply acts as that and you receive the input without needing to know the source of it. Mouse and keyboard events simply happen.
I could assume you talk abou the RFID scanner, did you get any further with that? If so, in what mode is it used? As keyboard? Then the keyboard input adds to what the normal keyboard input does, you won't be able to detect in a keypress from which USB port that came.
Chriss