Thanks to the existance of surface charges (namely, overall surface charges) existing on proteins, we can make sure of it to seperate proteins using a method known as electrophoresis. Electrophoresis, the migration of macroions as goverened by pH and electric fields, drifts proteins across a surface or differing pH levels, to the point where the pH of the surface allows for the protein to achieve its Isoelectric point (the point where its overall net charge is zero, thereby preventing any more movement caused by applied potentials). Noted is that the protein doesn't just instantly stop at this point, it has to oscillate a bit first as it slows down.
One of the great discoveries from electrophoresis is the differing of sickle cells from normal red blood cells. Despite not knowing the sequence of hemoglobin, Pauling et al found that there had been a change in the sequence as the titration curve of sickle cells was 1/5 of a pH unit different to normal hemoglobin. As it has since been found, a valine is replaced with glutamic acid, which is more acidic, causing the lowering of the titration curve.