hello. this is the ghost of kuroizetsubo talking from beyond the grave. i ctb'ed with full suspension hanging. it went quite well, actually. the afterlife is more peaceful than whatever chaotic mess the world of the living is.
In all seriousness, though, there is no way to know whether or not someone succeeded in CTB'ing, but if you keep up with a particular user's posts, they'll usually talk about which method they'll use, before posting a goodbye thread. Likely they will request for their account to be disabled or deleted, which is indicated by a crossed out line over their username.
Some CTB methods are proven to be more reliable than others, so it's usually a mix and match of the most convenient method for that particular person. Everyone has a different idea of what they consider to be the best suicide method. Look around and maybe you'll find one of such users.