Patchy colloidal particles offer a unique platform to explore and investigate spatially inhomogeneous colloidal interactions. In the present work we provide direct, in situ experimental observation at the single particle level on the colloidal interaction driven formation of a heterodimer, consisting of a patchy nanorod and a sphere. The gold nanorods employed during the experiments feature positively charged tip regions, while the side of the rods is covered by surface grafted PEG. The driving force of the assembly is the electric double layer interaction between the nanorod and the nanosphere possessing opposite surface charge. The great advantage of using gold heterodimers is that their optical scattering spectrum characteristics are known to be very sensitive to the actual structure of the assembly due to plasmon hybridization. This was exploited to determine the structure of the heterodimer in situ upon formation in aqueous medium. The results indicate, that despite the attractive patches being located at the nanorod tips, for the given particle pairs the spheres readily bind to the side region of the nanorods in the aqueous medium during the assembly. Finite element simulations of the electric double layer interaction reveal that this is the energetically favorable configuration for the given heterodimer. This work demonstrates the potential of in situ experiments performed at single particle level for the characterization of self-assembling structures, which can contribute to the development of rationally designed nanoscale building blocks.