What Makes Reverse Swing in Cricket Different From Conventional Swing?

I understand the basic idea of a cricket ball moving in the air, but reverse swing seems to behave differently from conventional swing, especially later in an innings. What changes in the ball’s condition and the bowling conditions make reverse swing possible?

Is there a simple way to distinguish the two by watching the ball, or do factors such as pace and the bowler’s release make that hard to judge from the stands?