How can I find overlapping bricks in LeoCAD?
I'm not aware of a way to do this in LeoCAD directly, but assuming that your overlapping brick is an exact duplicate (same element, same position, same rotation, etc) it will show up as a duplicate line in the LDraw file. Something as simple as this should show you what it is:

uniq -d model.ldr


You could also use any other tool for finding duplicate lines.

If you find a duplicate, it will look something like this:

1 4 -70 -48 -40 1 0 0 0 1 0 0 0 1 3005.DAT


The 3rd, 4th, and 5th items are the x,y, and z coordinates, which you can use to track down duplicate elements and remove them inside LeoCAD. Alternatively, you can remove all but one of the duplicate lines from the file directly.