From far, both style look very similar. I found the easiest way to identity the two bags is by looking at the handle.
Birkin has two handles attached to the front and back of the bag and they are longer than Kelly's handle.
Kelly only has one short handle on the top of the bag.
Here are more interesting facts about the two famous Hermes bags:
Named after Jane Birkin.
Sizes are 25, 30, 35, and 40 centimeters
It also comes in 45 and 50 centimeter as a travel bag
An average Birkin bag is created in 48 hours
More casual and can be use as a briefcase
It is easier to open than the Kelly
Victoria Beckham owns more than 100 Birkins ~ She is expecting a girl this summer, a lucky girl who will inherit all her mom's Birkins :p
Named after Grace Kelly
Sizes are 22 (Pochette) 25, 28, 32, and 35 cm
An average Kelly bag is created in 18 hours
A nice evening bag
So after all, do you prefer a Birkin or a Kelly?