After hearing out Batman's short comment, press LB to turn on the Evidence Scanner. Aim at the sniper rifle #1 and keep A pressed until you finish scanning the evidence #2.
Now you will have to talk with Joker, after which a countdown to the explosion of the bombs inside the bell tower will begin. Quickly approach one of the windows #1, lean the analog stick towards it and press A twice to make Batman jump through it, getting out of the explosive trap #2. Listen to the conversation with Alfred concerning the plan of tracking down Joker's signal, i.e. your next objective.