Sarah,
I don’t believe there is a simple block like “isTouching” in App Lab. I was curious to see if I could find anyone who had used “collision detection” in App Lab and I did find a Youtube tutorial on it where Bill Barnum explains how he programmed collision detection on his own and I found another (very long) Youtube tutorial on making the Pong game in App Lab that also involves collision detection.
So I guess anything is possible. 
Over on the CS Principles forum, there was also a good discussion on why App Lab doesn’t have collision detection and the net consensus is that students would have to program their own “isTouching” function to mimic that capacity that is already in Game Lab.
Anyone else have any ideas?
Mike