The Maine Coon can be recognized by a number of fairly distinct characteristics. (say, if your cat's a cross-breed... want to know if it might have some coon in it?)

Look for these traits:

Long Hair
A sort of "M" on the forehead defined by a darker hair color.
Often times the hair on the backside of the rear legs is particularly curly (like a poodles), because a) they clean themselves and b) the hair is of a certain texture/quality that it is easily "permed" by moisture and a cat's tongue ;)
Head-butting. The Maine Coon likes to head butt its owner/friends as a sort of "hello". I'm not sure if this is unique to Maine Coons though.

One of my Coon cats loves to have her head mauled... scratching her ears, head, neck sends her over the edge... But... she is a slobbering idiot.