How can I identify a patient of split personality or a psychopaths?

Careful history. Psychopaths are what are known in the dsm-iv-tr as individuals with antisocial personality disorder. This condition, unfortunately, is quite common, but very difficult to treat. "split personality" is a more controversial diagnosis and is known by dissociative identity disorder in the dsm. It is theorized to result from traumas that cause the patient to withdrawal from a very painful situation.
You can't. Making a psychiatric diagnosis is very difficult. Psychiatrists spend 4 years learning how to do it. The two diagnoses you listed are particularly difficult to make.