What are the visual differences between the ATR42 and the ATR72?
By Julien.scavini (Own work) [CC BY-SA 3.0], via Wikimedia Commons

The ATR72 is quite a bit longer than the ATR42, and has several more windows to allow for more passengers.

According to Wikipedia: 


  The ATR 72 was developed from the ATR 42 in order to increase the maximum seating capacity (from 48 to 78) by stretching the fuselage by 4.5 metres (15 ft), increasing the wingspan, adding more powerful engines, and increasing fuel capacity by approximately 10 percent.


So it's longer, and has larger wingspan. The stronger engines and larger fuel tanks can't be seen from the outside.