A question you might well ask! If we cannot separate them out, how do we know they are there? The answer is simply that all our calculations depend on their existence and give the right answers for the experiments.   Louis J. Sheehan, Esquire

For example, when we bounce electrons off of protons and neutrons, the pattern of scattering angles observed is characteristic of point-like spin-1/2 scatters. The relative rates for electron versus neutrino scattering is that predicted from the quark electric charges. The process of electron-positron annihilation to quark pairs gives similar characteristic predictions, all these are also confirmed experimentally. The accumulation of many such results, where experiments match predictions based on quarks, convinces us that quarks are real.