Black holes that have a star or gas cloud nearby are easy to detect due to the radiation that is produced as the particles fall in. Neutrinos are weakly interacting particles, but they do interact occasionally. The results of these interactions are the change of a proton to neutron in an atom. Billions of neutrinos from the sun pass through each human body every second. Two types of liquids that are suceptable to atomic change are chlorine and gallium.
Chlorine becomes argon when it interacts with electron neutrinos.
Gallium becomes germanium. By counting the number of atoms that change scientists are able to know the number of neutrino interactions that took place in a given time. Another way to detect the interactions with neutrinos is that Cherenkov radiation. Cherenkov radiation is a flash of light produced when charged particles move through water or ice at a speed greater than the speed of light in these substances. Only charged particles produce Cherenkov radiation, so neutrinos must interact with matter to produce an electron before the detector can count them.