Coronal holes are easy to spot by looking at the Sun through an x-ray telescope.
They appear as very dark areas that contrast with bright spots overlying sunspot groups. Hot gas around sunspots is captured by magnetic fields that rise up out of one sunspot and bend back to reconnect at another spot nearby. These glowing 'magnetic bottles' shine brightly at x-ray wavelengths. The magnetic fields around coronal holes are different. They extend far out into the Solar System and no one knows exactly where they reconnect.