Christ this forum is a good deal more juvenile and misguided than I thought. Feel free to appreciate the "fun" replies you've gotten but don't take them seriously. Most people here don't even understand what a sociopath really is despite their attempt to ape one.
Let me answer your question before I prattle on: Simply ask the person in question if they are a sociopath or not. If they deny it, respect their desire not to face the issue or the possibility that you're mistaken, and if they tell you that they are than it's fine. You lack etiquette the moment you presume you "know" they're some label or another. I am a sociopath Sacha. I read people very well. Eerily well. Boringly well. Still when I a believe i know the root of my friends behavior I don't shove it in their face like "I know you behave x and y way because of z" I ask "are you doing x and y because of z?" That's polite.
So shortly: Don't say you know. Just ask if they are one and respect their answer whether it's right or wrong (as right and wrong are fallacies of relativity)
The most recent take on sociopathy is not that sociopaths have no empathy, it's that we have the ability to turn off our empathy at will, almost as easily as we breath.
Not every narcissistic asshole is a sociopath. Not every person who plays mind games is a sociopath. Not every brilliant intellectual who's really GOOD at mind games and manipulation is a sociopath. If their negative behavior stems from insecurity or trauma as it does 99+% of the time then they aren't sociopaths, they are just pathetic overcompensating fools trying to wrest happiness from the misery and domination of others.
Take me: A sociopath who views hurting people as a cowardly, boring easy path to take. I enjoy making my friends laugh, improving lives around me, preserving and sustaining joy. It's much more challenging and stimulating. However on the rare (very) occasion that someone does... cross me, i devise punishment that's absolute and haunting. I don't particularly enjoy the act, I enjoy making my world conform to my own personal view of "right". I don't assume my "right" is right for everyone but I enforce it in my personal life, as it is my life.