I wanted to model the arms, but I hid the whole body so that I can only see one arm. Unfortunately, the mirror effect is not applied if everything else is hidden. How can I make the mirror effect work?
Press Alt + b while in Sculpt Mode to bring up the Region Clipping tool. A box can now be drawn to define the area you want to keep visible. Everything else will be hidden. The brush symmetry will still work on the hidden parts.
Press Alt + b again to cancel this mode. Note that it's not always easy to see where the edge of this region is, though you can get a sense of it by orbiting your view. As this tool takes a slice out of the visible area, the affected region actually exists within 3D space, meaning that if you are working up a sculpt, and happen to work your way past one of the clipped boundaries, that part of your sculpt will now fall into a masked region. If this happens, you'll simply need to reuse the tool, and define a new region.