How can distinguish that a nanoparticle is alloy or core-shell?

A core shell nanoparticle is composed by two phases, one in the core and another one covering it. The alloy ones are composed by only one.

The phases have different spectra for different wavelengths of electromagnetic radiation and electron beans. That means that the different phases are going to absorb different amounts of electrons at a given wavelength, controlled by the acceleration imposed to it by an electrical tension. This is what happens in electron microscopy.

In order to identify the two phases that behave differently with electrons, one possibility would be using transmission electron microscopy (TEM). TEM places the electron emitter and the detector on opposite sides of the sample. That means that the electrons must propagate through the sample, thus being absorbed in different intensities for different phases. That you result in a core shell structure showcased as a core covered with a material with a different visual aspect. The alloy one will have just one aspect. An example is the microscopy below, where the one on the left (a) is an alloy structure and the one on the right (b) in a core shell structure.