Characterisation of gold-silver alloy nanoparticle
I Think you are in right track using UV-vis spectroscopy as a tool to identify your alloys. Other than UV-vis spectroscopy, researchers around the world have used many other analytical techniques to evaluate the synthesized nanomaterials, including X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and etc. (For review of these techniques: Read Ref.1). 

According to your notes, it seems like your main concern is how to confirm that your samples are alloys. UV-vis spectroscopy is a very useful and powerful tool in that regards, and reliable technique for the primary characterization of synthesized nanoparticles as well. Silver absorbs around $\pu{400 nm}$ and gold around $\pu{550 nm}$, yet these values vary depends on particle sizes of the dispersion (See Figure 1A &amp; 1B, respectively). For example, absorption spectra calculated using the quasi-static approximation for silver and gold nanospheres with diameters $\mathrm{D} = 26$ and $\pu{20 nm}$, respectively, have shown absorption maxima (surface plasmon resonance wavelength $\lambda_R$) at ~$\pu{375 nm}$  for $\ce{Ag}$ and ~$\pu{525 nm}$  for $\ce{Au}$ in water (Ref.2). This reference also discussed the behavior of $\lambda_R$ as a function of the dielectric constant of the surrounding matrix ($\epsilon_m$), which is 1.77 for water. 



This phenomenon was used to the best in identifying $\ce{Ag-Au}$ alloys in various compositions in an article (Ref.3), abstract of which states:


  Gold−silver alloy nanoparticles with varying mole fractions are prepared in aqueous solution by the co-reduction of chlorauric acid $\ce{HAuCl4}$ and silver nitrate $\ce{AgNO3}$ with sodium citrate. As the optical absorption spectra of their solutions show only one plasmon absorption it is concluded that mixing of gold and silver leads to a homogeneous formation of alloy nanoparticles. The maximum of the plasmon band blue-shifts linearly with increasing silver content. This fact cannot be explained by a simple linear combination of the dielectric constants of gold and silver within the Mie theory. On the other hand, the extinction coefficient is found to decrease exponentially rather than linearly with increasing gold mole fraction $x_{\ce{Au}}$. Furthermore, the size distribution of the alloy nanoparticles is examined using transmission electron microscopy (TEM). High-resolution TEM (HRTEM) also confirms the formation of homogeneous gold−silver alloy nanocrystals.


According to this reference, only one plasmon band for particular alloy (Two bands would be expected for the case of a mixture of gold and silver nanoparticles). The band shows a blue shift with increasing $\ce{Ag}$ amount in the alloy (See Figure 2) and a linear relationship with increasing gold mole fraction, $x_{\ce{Au}}$ (See the insert in Figure 2):





References:


X.-F. Zhang, Z.-G. Liu, W. Shen, S. Gurunathan, “Silver Nanoparticles: Synthesis, Characterization, Properties, Applications, and Therapeutic Approaches,” Intl. J. Molecular Sci. 2016, 17, 1534 (34 pages) (doi:10.3390/ijms17091534).
F. Vallée, “Chapter 7: Optical Properties of Metallic Nanoparticles,” In Nanomaterials and Nanochemistry; C. Bréchignac, P. Houdy, M. Lahmani, Eds.; Springer-Verlag: Berlin, Germany, pp. 197-228, 2007.
S. Link, Z. L. Wang, M. A. El-Sayed, “Alloy Formation of Gold−Silver Nanoparticles and the Dependence of the Plasmon Absorption on Their Composition,” J. Phys. Chem. B 1999, 103(18), 3529–3533 (DOI: 10.1021/jp990387w).