You do that by looking over the board's own diagram. For most boards the small block or pins near one edge of the board are marked with small print with things like PLED for power RSET for the reset button. HLED or HDLED for the hard drive activity led. The wires coming in from the front panel should also have abbreciations on the small plug or tag attached like PS, RS, HD to distinguish those. They will also have a plus and minus sign for lead and ground that you simply match up with the pins on the board.