Indium nitride InN exists in two different structural phases, the equilibrium wurtzite w and the metastable zinc-blende zb phases. It is of scientific interest and practical relevance to examine the crystal structure of the epifilms during growth. In this paper, we use Patterson function inversion of low-energy electron diffraction I-V curves to reveal the preferential formation of zinc-blende InN wetting layer during the Stranski-Krastanov growth on GaN0001. For three-dimensional islands nucleated afterwards on top of the wetting layer and for thick InN films, the equilibrium wurtzite structure is observed instead. This in situ revelation of the InN lattice structure is confirmed by ex situ transmission electron microscopy studies. Finally, the formation of zb-InN layer on w-GaN is explained in terms of the strain in the system.