Can the eye distinguish between pure and composite colours and how do we measure this?
Yes, we do perceive a composite spectrum in a way which is analogous to how we perceive the corresponding single-peak spectrum. Human beings have trichromatic vision, meaning that there are 3 types of specialized cells in our eyes for seeing color. Each one has a different color of light which produces peak stimulus, although they all perceive a range of colors:


(the axes are wavelength and normalized response)

None of the color-sensing cells has a peak color of yellow. In order to perceive something as being yellow it either has to be pure yellow or stimulate the 3 types of color sensing cells in a very similar way as pure yellow would. However, if you took very careful readings of the output signals from the color sensing cells you might see some subtle differences in the exact waveforms of the responses.