According to Jesus,'Immediately after the tribulation of those days the sun will be darkened, and the moon will not give its light; the stars will fall from heaven, and the powers of the heavens will be shaken. Then the sign of the Son of Man will appear in heaven, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of heaven with power and great glory. And He will send His angels with a great sound of a trumpet, and they will gather together His elect from the four winds, from one end of heaven to the other' (Matthew 24:29-31).
To us, this is the best indicator as to when the Rapture will happen. So as soon as a time of great tribulation ends, just keep watching for the sun and moon to be darkened and the stars to fail to shine. Once that happens, you can know for a certainly that Jesus will be coming very very soon.