Jesus was clear that concerning His coming there aren't signs. Instead He says it will be as in the time of Noah, people going about their business, getting married, having families, when without warning the flood came. He says two will work the grindstone, one taken and one left; two will be working the field, one taken and one left.
If you want to know what Jesus means about being watchful, don't stop reading, keep reading. Read the rest of the Olivet Discourse in Matthew 25. Who are the unwise virgins? The ones who failed to bring enough oil to last through the night (they did not endure through the wait); who is the faithless servant? The one who, instead of taking what he was given and doing something with it, buried it in the ground and waited around until the master came. When the King comes to Judge, there will be those on His right and on His left, what does He say to those on His right? What does He say to those on His left?
Notice that absolutely none of this has to do with trying to make predictions. The signs the Lord gives come before He talks about His return, and are the signs of the destruction of the Temple, fulfilled during the Jewish-Roman War. Near the very start of the Discourse the Lord warns against listening to people claiming to know things, "If someone says He is in the desert, do not believe them", He says that when we see wars and rumors of wars, natural disasters, or kingdom rise against kingdom, that this isn't a sign of the end, these are birthpangs.
You know where else that talk of birthpangs shows up? In Romans 8, talking about the pain of creation currently suffering under the futility of death, longing for the day of resurrection. That's what birthpangs are--a world that is hurting because there is sin and death. That's why there is war, that is why there are rumors of war, that is why their are earthquakes and other natural disasters, that is why nations rise against nation, and kingdoms against kingdom--because we live in a fallen and broken world of sin and death.
Read the entire Olivet Discourse, and read it carefully. Let the text speak for itself.
-CryptoLutheran