What about online posters who warn about online ministries? lol, just kidding, couldn't resist.
I would say it's not online ministries being online per se that's the issue. The internet is just a medium. Are we still preaching on fisherman's boats, as Christ Himself did? Not necessarily, as times have moved on, there's another kind of net now!
I would say you know a True Preacher of Christ (1) if he glorifies Christ as much as he can (2) he does not love money (3) there are no scandals etc associated with him, i.e. his walk is blameless or close to it, and (4) he speaks as little as possible about himself. A ministry should be all about gaining glory for Christ in as many hearts as possible, i.e. either if those hearts never knew Christ, then to help them know Him, or, if they know Him, then to help them grow in His love and grace every day.
Just my personal thoughts. God Bless.