What Are "Otherkin"?
The premise is simple...it's the same for vampires as it is faeries, were, angels, demons, etc...

Non-human souls being born into human bodies/families. This can also get fringe meaning by said non-human soul having children, who while human, posess an ingrained level of magick/power similar in nature to that of a full otherkin. That's the herditary side of things. Mostly seems to happen with fae and is generally referred to with terms like "Fae gifts" (the natural in-born magick of the fae, but not actually posessing a fae soul), or "bloodkin" (children/family of those who are otherkin that have either gifts themselves or have just learned the realities of it all because of the existance of otherkin in their lives)

The thing is, it's an overused term just as much as vampire is. Everyone flocks to the term because everyone has some reason or another to think they are not-quite-human. By the strictest definition though, unless you actually for a fact, have a non-human soul, you can't really call yourself "otherkin."

Now...what is speculated is this. There are multiple realms/dimensions/realities/etc. in which there are other races. Somehow-for unknown reasons-the souls of those races can cross to THIS realm/dimention/realiy/etc. and be born as humans. But, they are really not human. Most posess magickal gifts, talent, powers, needs, and memories that move WAY past that of "normal" humans. Most have a deep longing to go "home" but don't know what or where it is they are longing for. Most feel very out of touch or place with modern life, finding themselves different from a very early age and possibly even stating things as young children that adults pass off as "fantasy." Many otherkin grow up and grow out of these memories and magicks because they are EXPECTED TO. Sometimes, they will come to a point much later in life where they re-awaken that part of themselves. Most however, who fall "asleep" (meaning, they completely forget their true nature and burry their magick and memories somewhere they don't even know how to find them anymore) don't ever wake up.

At one point in time, I believe that the veils/barriers/mists/etc. between the worlds (realms/dimentions/whatever you want to call them) were illusionary and otherkin could walk along the earth without having to first assume human bodies. I believe that at one point, what we now consider "fairy tales" were really just otherkin in their natural form. But, as humanity grew, changed, learned science over belief, the barriers grew and the possibility faded. Now, you can't have an dragon be in this world in their true form for example, because no one could handle the existance of one.

It's hard on the net to find things that are real. Most people simply like faeries and dragons and such but have no real knowledge or fact. They are not otherkin, nor do they truly know what it means to BE otherkin. I don't think there is any particular "test" other then most know as children what they are and they simply have to surive with that knowledge in tact in a world that tells them to dismiss it. Some people claim they can see the difference in an otherkin's aura, but I've never pursued this in person. Some have strong astral ability and take their natural form there.
 

Common Traits/Talents
Very Energy and more often Nautre Aware.
    --Ability to "talk" to the wind, water, trees, etc.
    --Ability to "hear" them reply
    --A deeper, different, and more instense connection to the earth
    --A deeper, different, and more intense connection to the magickal/astral/dream/etc.
    --A strong connection with the stars, sun, moon, or celestial is common
 

Most otherkin DO tend to have higher then average IQ's
    --Mostly believed to be an extention of using more of one's mind and potential.
    --Strong abilities in one field, poor in others (generally more liberal and arts minded)
    --A strong sense of knowledge and need to learn (probably on ones own time and in their own way though)
 

Being overly sensitive to the "normal" senses
    --Hearing light, seeing smells. Sounds weird, but seems oddly universal.
    --Loving or hating sunlight-finding it either charging or draining.
    --Whines from electronics easily heard, perfumes easily smelled, etc.
    --Strong loves and hates of food and flavors
    --The ability to "get lost in" or "drown" in pleasing colors
    --To be emotionally moved by a site of Beauty
 

Empathy and/or mild Telepathy.
    --This can sometimes gets to be a literal headache.
    --May show up in childhood and be repressed for awhile or kept secret though school years.
    --Just "knowing" when someone lies or tells the truth
    --Getting a "feeling" about the nature of a person and their intentions
    --Ability to convey a thought/emotion/need to another unspoken
    --It seems like a natural extension of oneself. (I know I'm blind when I can't read someone or if I'm in such a large crowd that I get overloaded.
 

Seperation and Feeling Out of Touch
    --Sometimes even a very "protective" feeling towards others?
    --The counter of this is feeling very "dominant" towards others
    --Feeling "one step behind" of the crowd
    --Having a speeding or slowing of Time