How will we know when the rapture is happening?
1 Thes. 4:13-18 provides the best description of the rapture. Jesus will descend with a shout with the voice of the archangel and a trumpet blast of God. The dead in Christ will all rise first, followed by all who are alive and remain.
The shout and trumpet blast should make it clear that the rapture is happening to any who are looking for His coming (James 2:11-15). Our bodies will be translated to a form like Christ's (post resurrection), and we will rise to meet Him in the air. Any open casket funerals of Christians in progress will be different to say the least.
It is assumed that our earthly clothes will be left behind and replaced by something heaven appropriate. So there should be direct evidence to the world that we were there and that we then were gone if we are awake and out and about business or school or shopping.