How do you tell a sociopath that they are one?

Unless a sociopath is professionally diagnosed, you cannot tell that person that he/she is a sociopath. And the chances of one going to get a diagnosis are virtually nil. And DO NOT tell others that you think he/she is a sociopath in case it gets back to the person. Since a sociopath has no conscience, and since only self matters to a sociopath, he/she can be vindictive and even litigious when it comes to being told about his/her condition. There is no therapy to "fix" this condition - just protect yourself and those around you. Get the person out of your life as cleanly and quickly as possible.