I can't tell if my uggs are fake! help?

i got uggs for christmas and i think they are fake. i looked at my friends which were the same size and color and they looked different. they were about an 1.5 inches shorter, they had i little different color, but still an ugg sign on the back but thats a different color too. and a bunch of other things are different too! i just dont think mine are the real ones. which site is the most reliable to get real one, and which arent??? how is the best ways to tell that they are different?