That is true. Only an enlightened one would be able to tell with certainty. Then we would have to decide if the one declaring the Dalai Lama enlightened or un-enlightened is enlightened. Others can only suspect or assume. The Dalai Lama himself says that he is not enlightened, and people suspect he is is lying. The assumption is that he is lying out of compassion so people do not feel inferior to him. People insist on putting him on the pedestal of enlightenment for their own purposes. After all, who would they be if they followed and adored an un-enlightened one?
To say that "enlightenment is beyond human comprehension" is true, but enlightenment is not beyond humans!!! An important distinction. It is quite possible that the Dalai Lama is not enlightened, and the monk cooking his food or sweeping outside of his door - is.