You can tell them by the different color wire in each. If I'm reading the wiring diagram right, the e36 harness colors on pin 1 of each connector are 1 - white, 2 - red, 3 - yellow, 4 - blue. The other wires will be brown and green.
Make sure you put them 1-2-3-4 from the radiator to the firewall, matching the cylinder numbers. You'd be surprised how many people plug them in according to firing order.