Quarks can not observed directly in isolation so that they only can exist in the form of colorless hadron.
So we may suggest some questions like "Is quark actually exist?", "Is it just mathematical tool for describing hadrons?".
Fortunately I heard that deep inelastic scattering experiment provided the evidence of the reality of quarks.
What experimental results of DIS prove the fact that quarks are actually exist?
The existence of quarks was established in a series of particle accelerator experiments carried out from the mid-1960s to the mid-1970s. The earliest experiments happened at the SLAC facility at Stanford, where extremely high energy electrons were fired at protons; some of the electrons recoiled at large angles and from that data it was established that the protons behaved as if there were three very tiny and hard things running around inside each of them.
As such, those things (which were named "quarks") were not just mathematical abstractions but had a real existence.
An excellent and thorough historical account of this is given in Riordan's book, "The Hunting Of The Quark".
