1. You're a blond...
If you have blond hair or have recently died your hair blond then it's a dead give away that you are becoming a Lannister.
2. You have a resting bitch face
Your face just doesn't smile or when it does it is only out of your own sarcasm. Your friends think your face is punchable, and punchable faces are very Lannister.
3. You're have more in common with your uncle than you think...
If you've recently discovered that you're uncle is more than just your uncle, then you are on the right track to becoming a Lannister.
4. You enjoy a nice beverage...
Even at midday, if you're a Lannister, you start popping bottles.
5. …like, seriously, if you're a Lannister, you'll always have a glass in hand.
Good thing about a Lannister is they know how to party. So if you love to party, you might a Lannister.
6. You're a real dick
If you treat people badly for fun, one you're a dick, and two you're becoming a Lannister.
7. You're so vain
Lannisters love checking themselves out in gold plates. So if you eat off a gold plate and check yourself out in it, you're a Lannister.
8. You're sneaky
If you find yourself sneaking around the house late at night for a treat, planning something evil or even manipulating someone to do things they don't want to do something they don't want to do, then you are becoming a Lannister.
9. The weddings you attend are crazy...
Did someone break their leg during the bouquet toss, threaten warfare on another house, or choke to death on some wine? Then you may have just been to a Lannister wedding.
10. You've begun repaying your debts.
If you've begun repaying your friends for those smaller coats then you are definitively becoming a Lannister. In Game of Thrones, the Lannister words after all are "A Lannister always repays his debts".