is a small earthquake that often precedes a major earthquake

The mainshock is the largest earthquake in a sequence. A foreshock is a smaller earthquake that precedes the mainshock, and an aftershock is a smaller earthquake that occurs after the mainshock.

An aftershock is a small earthquake that follows the main earthquake, while a foreshock is a small earthquake that often precedes, leads to, a major earthquake.

In any series of earthquakes in the same area, the one with the largest magnitude is called the mainshock. Anything before this is called a foreshock and anything after is called an aftershock.

No, they can't tell that it is a foreshock until a larger earthquake happens. (The largest main earthquake is called the mainshock)