"That remains to be seen" is an idiomatic expression used in response to a statement. It is an expression of skepticism and doubt related to two sayings: "seeing is believing," and "don't believe everything you hear."
Example: "Analysts predict gas prices will actually start falling toward the end of May, as refineries increase production and more gas becomes available. That remains to be seen: Many analysts failed to predict the prices drivers are paying now, caught off-guard by surging oil prices." Source