"You can't observe them directly"
By that logic, you can't observe anything directly - all observations are records of something interacting with something else...
Even light - that's the interaction between a photon and the cells in your eye.
In the early days of particle physics - the interaction between a particle and the (supersaturated) gas in a cloud chamber, forming (essentially) a vapour trail... which is observed by... light.
The first observations for neutrinos - the interaction between the neutrino and by the (very occasional) atoms that happened to be hit by one, gaining the energy required to, at a later point, emit a photon of light.
(...but, before people start thinking there's a theme here...)
The experiments Ray Davis performed down in a mine involved the transmutation of Chlorine into (a radioactive isotope of) Argon, which could be collected and counted.
(Mine's the one with the Physics Degree in the pocket...)