Journal article: PRL: http://prl.aps.org/abstract/PRL/v104/i12/e123605
The basic idea of the experiment is simple. Take two objects that emit single photons that are identical to each other. Detect these photons after they have been mixed together using a beam splitter. If they are identical, both photons will appear on the same output port of the beam splitter and set off the same detector. If they are different, they can appear on different output ports and set off different detectors.
As the wavelength difference between the two photons varies, the statistics logged by the detectors reveal whether the photons can be usefully considered "distinguishable."