http://en.wikipedia.org/wiki/Hare_Psychopathy_Checklist, and before that, a very strong gut feeling. Too many things click together: the overall toxicity of a person, his desire to belittle / denigrate many people around him (a waitress in a cafe, contributors to an internet community project, a non-psychopath competitor etc.), his grandiose self-image (he literally designated himself as 'elite'), threats of violence and criminal leanings, always hogging the spotlight in social settings, "friendships" that change to enmity overnight, et cetera et cetera.