How do i know i have a real maine coon kitten?

How do you know if you are getting a real maine coon kitten? I went to see some kittens today that the owner is claiming are maine coon kittens. She showed me who the dad was supposedly and he really does look like a maine coon, she did not show me the mom though, but the kittens don't really look maine coon. They are 8 weeks old. The owner is selling them for 10 dollar and she claims that they have an "M" on their forehead and she says that's an indication that they are maine coon. I'm not to sure about it, I don't really believe it. So my question is are there any certain traits or characteristics other than the long hair?