One of the things they're doing is calculating what it's orbit would have to be to hit the Earth, and where it would have had to have been on its last orbit to be in that orbit

So they can look at any astronomical images of that part of the sky from then and see if it's in the right place

If they find images of the right part of the sky at the right time and the asteroid is not in it, they know it's not on an orbit that will hit the Earth in 2032