gmwilliams, love this question!
1) Quote poetry or scriptures with a maniacal look in one's eyes.
2) A maniacal laugh!
3) Surround self with minions.
4) An eye patch or big mustache.
5) Black robes, capes, clothing.
6) Have some kind of a facial tick.
7) Have a pet of some kind to show affection to.
8) Be angry with a vendetta against a person or organization.
9) Live in a secret hideout or dark castle.
10) Have specialized knowledge (mad scientist) or super-power.