Mendel Shneerson was not the messiah -- he did not complete the tasks of the Messiah, so just like Jesus, he is ruled out of the competition. There are some who love him so much, that they are having a hard time accepting that he is dead, just like what happened with Jesus. Chabadniks are wonderful people, but any who still claim that Shneerson is the messiah are engaging in heresy. The do not worship the Rebbe as a god, so it is not some other religion, and they are not apostate, the way that Jews who worship Jesus are apostate to us.
The basic answer is that we will know the messiah because he will do the tasks that are prophesied for the Messiah to do, such as usher in an era or world peace, such as bring all the Jews back to the Land, such as rule from Jerusalem.