Disassociate Identity Disorder (DID), formerly known as Multiple Personality Disorder, is a kind of dissociative disorder wherein critical parts of one's personality are split apart, due mostly to severe stress and experienced trauma. That person would therefore exhibit two or more distinct personalities, also known as "alters." Many studies have found that multiple personality disorder has been a coping mechanism for its sufferers; by having more than one personality, they can best avoid stress and eliminate anxiety by isolating that traumatic experience to only one part of their personality, leaving the other parts of their personality free.
The different personalities held by a DID sufferer each have different sets of likes and dislikes and different ways of reacting to situations. That's why one of the earliest symptoms that a person has DID is if he would show inexplicably different emotions and reactions to situations. Also, somebody with DID would have totally no idea of what one of his personalities would do: for example, if personality A would buy new clothes, personality B would be surprised at finding new clothes in his closet. So this is another way of telling that a person has DID: more than forgetfulness, he exhibits total lack of knowledge of what he has done previously, or amnesia.
The list of symptoms of DID would include depression, unexplainable phobias, frequent panic attacks, personality antisocial (or low capacity to cope during social gatherings), or personality avoidant (preferring to be alone and having great difficulty going out of the house), hearing voices inside one's head, and physical pain. Sometimes, a DID sufferer would become dependent on self-medication, drugs or alcohol in order to try to relieve himself of the symptoms. This will result in a cycle of even more depression and exhibition of the symptoms of DID.
However, one of the prime difficulties of diagnosing DID is that some patients would imagine that they have DID and would identify simple negative social behavior, forgetfulness or different idiosyncrasies as possible DID symptoms. That's because DID is a very popular disorder that has been featured in many psychological movies and novels. Also popular, but less symptomatic, are borderline personality disorders and borderline disorders, wherein sufferers lead seemingly normal lives, but with underlying maladaptive personality traits that disable them from functioning well as members of society. Borderline personality disorder (BPD) may be associated in some way to DID, since BPD sufferers have no secure sense of identity, and they rely too heavily on external factors - such as relationships - in order to find security and well-being. However, this reliance usually proves destructive to relationships, and they show intense feelings of jealousy and distrust, and they also have difficulty controlling their emotions and anger. Some DID sufferers, like BPD patients, would engage in self-mutilating activities such as cutting or hurting themselves. A DID patient is also often diagnosed to have BPD as well.
In diagnosing DID, professionals would give mental health interview questions, since there are no physical tests (like blood tests) that would definitively produce clear-cut results.
If you suspect that you have a personality disorder or even a DID, you could take online personality quizzes from websites such as 4degreez.com, quizmoz.com, and wellsphere.com/wellpage/Tests-for-Multiple Personality Disorder. Do remember that despite taking these quizzes, it's highly important that you seek valid professional opinion as well.