As others have said, we can’t know for sure. But there is an indication. Asteroids orbit in stable orbits, for the most part. They aren’t just randomly leaving their orbits and attacking the earth, at least not at this stage of the solar system’s evolution.

Here’s what we think happened. A collision between two asteroids launched a chunk of one into a different orbit within the asteroid belt. Now, there are certain radii in the asteroid belt where an orbiting body interacts with the gravity of Jupiter. This is called an orbital resonance. Jupiter will dump energy into a body in these radii and pump its orbit up. Eventually, the object’s orbit will become more, what we call, eccentric. This means it becomes more elliptical. If the axis of that ellipse crosses the orbit of an inner planet, then the two can collide.

But anyway, to answer your question, we know from the physics how much energy is being pumped into an asteroid that causes it to shift its orbit. We can do those calculations and combine them with what we know about the average velocity of an asteroid in the asteroid belt and get an estimate of the speed of the impactor’s approach.