In Matthew 24, the disciples asked Jesus directly: "What shall be the sign of thy coming, and of the end of the world?" Jesus responded with a detailed list of signs that would characterize the time leading up to His return. He warned of false christs, wars and rumours of wars, nation rising against nation, famines, pestilences, and earthquakes in divers places. He called these "the beginning of sorrows" — birth pains that would increase in frequency and intensity.
Jesus also warned of great persecution of believers, widespread deception by false prophets, and the love of many growing cold because iniquity would abound. He declared that the gospel of the kingdom would be preached in all the world as a witness unto all nations before the end comes. These signs describe a world in escalating turmoil, exactly as we observe today.
It is important to distinguish between signs of the tribulation (which are specifically for that seven-year period) and general conditions that indicate the tribulation — and thus the rapture — is drawing near. The rapture itself is signless, but the shadow of coming tribulation events is already visible.