Explain what "identify" means. You can of course just make a binary image of where that criteria is true like this:
zIsInRange = (z >= minZ) & (z <= maxZ);
But this binary image could have several blobs on it. That's why I ask you what "identify" means to you. Normally if you had the Image Processing Toolbox, you'd call bwlabel() or bwconncomp().