Just under your left eye.

Drucker:
Adam, Adam, Adam. He's not the clone. You are. [Adam laughs] Drucker:
I want you to ask yourself something. Do you remember anything after being scanned by my bodyguard? [Flashback plays] Drucker:
Do you actually remember changing places with your friend? You know, the salesman at the retail shop thought it was odd that you came in twice. Asked the same questions, twice. [Another flashback plays] Drucker:
Your shaving cut? Easily reproduced. So is the scar from your war wound. Adam Gibson:
I know who I am. Drucker:
I know you do. Talia, how many times have you been cloned? Talia:
I've lost count. Drucker:
There's one way to tell. [to Talia] Drucker:
Show him. [to Adam] Drucker:
It's the only way to keep track of what generation a clone is. [Talia pulls down her lower eyelid, revealing four dots inside] Drucker:
See? Four dots means she's been cloned four times. Come on. Let's take a look at yours. Come take a look in the mirror. Let's find out if you are who you think You are. [Adam goes to the mirror] Drucker:
Just under your left eye. [Adam pulls down his left lower eyelid, revealing a single dot]