Not sure what the justification is for saying Y functions 'psychologically' as a primary color. It would be a secondary color (R + G) in terms of the light spectrum, and (as you noted) our cones are directly most sensitive to RGB.
To answer the question, as qualia cannot be compared from person to person, there is in principle no way to show that they differ (or are the same) objectively. A sort of relative comparison of qualia within an individual (as you propose) will also not get you there, as it may be that the individual is simply more sensitive to changes along the gradient, not that they are experiencing a new color, and that's assuming we are able to use an objective comparison scale which we aren't.
To answer the question, as qualia cannot be compared from person to person, there is in principle no way to show that they differ (or are the same) objectively. A sort of relative comparison of qualia within an individual (as you propose) will also not get you there, as it may be that the individual is simply more sensitive to changes along the gradient, not that they are experiencing a new color, and that's assuming we are able to use an objective comparison scale which we aren't.