Re: able to understand when a person is being sarcastic
I'd say that the people who use tags like /s and not don't get sarcasm either, since doing that negates the effect.
It is quite difficult for both machines and humans to distinguish sarcasm without context.
Perhaps therein lies the key, because nothing exists without context. Someone who doesn't get the context is probably not aware of their social or other environment in which the sarcasm is presented.