There are many ways to detect things that might hit Earth. For larger objects like asteroids, we can search with telescopes. We take images every night and look for objects that move across the sky. If we can see the object in several images taken on different nights, we can calculate an orbit and then predict whether it will hit Earth. It's hard to detect small asteroids this way because they are very faint and so you need a big telescope or a long exposure time with your camera.
Most meteors that we see are caused by pieces of rock and dust that are much, much smaller than asteroids (only pea-sized or smaller!), so we can't detect them until they start to decend through our atmosphere. These smaller objects are sometimes observed by using radar. When a meteorite is travelling through the atmopshere, it heats the surrounding atoms and ionizes them. This creates an ion-trail in the atmosphere. If we send a radar pulse from the ground, it will be reflected from these trails of ions, and we can tell there was a meteorite there.
Another way to see incoming meteorites is to have an all-sky camera or small telescope that takes pictures of the sky continuously looking for the bright trails that meteorites produce when they fall through the atmosphere. There are a lot of cameras like this that search for meteors, many of them run by amatuer astronomers (and even schools or science clubs) that hope to find meteorites that might fall close by and remain intact.
This page was last updated on July 18, 2015.