There are a couple of ways to tell the difference depending on whether its day or night.
During the day on the ground the A320 sits higher than the 73, on approach the 320's gear 'dangles' for want of a better word whereas the 73's gear is much closer to the body of the aircraft.
If your getting really simple then the 73 has more of a pointy nose, and the Airbus is more rounded.
At night the main way to tell the difference are the strobes. The A320 has 2 strobe lights on each wing as redundancy so when they're both working you get a double flash instead of a single one on the 737.