If your date shows up wearing contact lenses that make his/her eyes look demonic (and it's not Halloween) take it as a sign that their tastes may run a little dark. That being said, these contacts are kind of cool. 

If the guy or gal you're crushing on has a sinister tattoo, take a moment to think, this person has permanently branded themself with an evil symbol. Although it may simply be for cool points or a show of individuality, there's also the possibility that they feel some allegiance to the dark side. Don't run away quite yet, though; only getting to know the person a little better will the answer that question for you.

An excessive amount of piercings (nowadays people have been known to pierce their eyeballs and even their skulls) is an indication that the person is a fan of pain. Some people use pain as a way of escaping the turmoil in their minds. This is by no means a healthy practice. You may see excessive piercings in combination with goth makeup and/or dress. Again, this is how some people choose to express themselves, but it is also frequently a sign of a personality disorder. They may actively seek out painful stimuli and experiences to satisfy a pathological need. Who is to say that those experiences won't be extended to you through risky behavior and aggressive sexual practices? 

If you go to your date's house and discover a room painted black or blood red, a light bulb should immediately go on in your head. There may be candles, skulls, a collection of violent music, videos or video games. Sometimes one might find satanic books and/or knives. If this doesn't raise a red flag then nothing will.

Devil worshipers sometimes acquire a taste for blood, whether it be human or animal. They may have a tendency to order their meat rear; this in of itself is not cause for concern, but if the person has containers of blood in their fridge (animal blood can be purchased from butcher shops) think twice about continuing to date them.