If the webcam connector is really USB (which you can check using USBview.exe utility), it should have GND, D+, D-, and VBUS.
The GND pin must be easy to determine with any continuity tester. The VBUS must be 5V, which should be also easy to determine by any DMM.
The D+/D- wires you can simply connect at random, and see if it works. If not, swap them. Alternatively, if you have any minimal oscilloscope, you can check which wire comes to 3V state first (for 100-400ms) after power-up (when the webcam is connected); it will be your D+.