Had Muhammad(sa) really been a Messenger, he would not have responded so helplessly to questions put to him, and would not have said, 'Only God knows'; which implies that he did not know. Nor would he have lied about the number of Ashab-e-Kahaf,1 nor would he have said that the Sun sets in a murky pool, for the Sun is 90 million times larger than the earth and it can not possibly hide in a pool of murky water.
1 People of the Cave. [Publishers]