Identity could be viewed in many ways, but the first thing that comes to mind for me would be specific elements of a subject that give it identity. For example, certain automobiles get their identity from smaller elements such as a hood ornament or a body curve. People gain identity from things they have.
Look for things that support identity rather than blatant displays such as a face.
Have fun with this... I would love to see your results :)
Fingerprint - As was said
Phrenology - If you have a friend with a shaved head it could be an interesting take on personal identity
Mugshot - left, right, and center view of a person in front of a height chart maybe holding a plaque with his/her name, date, and a number
A mask - Concealing or alternate identities
Clothing - The identity one gives to the world through them. Perhaps a group shot of different people in dramatically different styles.
A Flag - National identity
