Start off by launching the Evidence Scanner by pressing LB. Aim at the hole in the window through which Joker's bullet came #1. Approach it and keep A pressed until the scanning ends #2.
Now you need to find the place in which the bullet has ended its flight. Examine the floor #1. Just like before, you need to aim at the trace and keep A pressed until the scanning ends #2. You should now notice that the bullets' trajectory is visible in detective more, though in order to determine the exact spot from which it was fired you will have to go outside.