Off the top of my head I don't really remember, though Brian Greene does devote a whole chapter to the question of detecting other universes in his book, so that might be a good place to look further. At the moment though, this doesn't really bother me. For example, Giordano Bruno suggested the existence of exoplanets in 1584, and certainly he could not have predicted the modern techniques the Kepler telescope uses to detect them! It may be centuries before we have the theory or technology to determine if other universes are detectable or if they exist. A long time to wait perhaps, but this should not be a reason for ruling them out (though of course, solid evidence would be required before a multiverse theory could be accepted).