Jesus replied that His critics did not need a special sign since many things pointed to His being the Messiah. They could read the sky well enough to predict what the weather would be like soon. However they could not read what was happening in their midst well enough to know that their Messiah had appeared. The proof that they could not discern the signs of the times was that they asked for a sign. [source][source][source]
"It is surprising that in a wide variety of different fields of knowledge human beings can be so knowledgeable and perceptive, yet in the realm of the knowledge of God exist in such darkness. The explanation of the latter sad state is not to be found in a lack of intellectual ability-no more for the Pharisees and Sadducees than for today. The evidence is there, examinable and understandable for those who are open to it and who welcome it. The issue in the knowledge of God is not intellect but receptivity." [1][source]
What were the signs of the times that Israel"s religious leaders failed to read? John the Baptist"s appearance and preaching were two. John had told these leaders that he was the fulfillment of Isaiah"s prophecy of Messiah"s forerunner ( Isaiah 40:3; Matthew 3:1-12). [2] Jesus had also identified John as the forerunner ( Matthew 11:14). Jesus" works were another sign that the King had arrived, and Jesus had pointed this out ( Matthew 12:28). Finally the prophecy of Daniel"s69 weeks should have alerted these students of the Old Testament to the fact that Messiah"s appearance was near ( Daniel 9:25-26; cf. John 5:30-47; John 8:12-20). [source][source][source]