We have investigated a set of InxGa1-xAs/GaAs quantum dot structures grown by solid source molecular beam epitaxy for a wide range of In content ranging from 30% to pure InAs/GaAs dots. It is well known that in a self-assembled growth mode, the dots are formed on a thin layer of the InxGa1-xAs material, which is called wetting layer (WL). The WL thickness is driven by the strain, i.e., lattice mismatch between the layer and substrate materials. Usually, the WL quantum well is not optically active in emission type of experiments (the whole radiative recombination goes through the dot states) and even if so, the heavy hole ground state transition is probed only. In order to detect all the possible transitions, e.g., transitions related to light hole and possible higher order heavy hole states (including those transitions, which are nominally parity forbidden), we have used modulation spectroscopy in a form of photomodulated reflectivity measurements. This is an absorptionlike method, which has been proven to be highly sensitive to even very low intensity transitions in low-dimensional semiconductor structures. The aim of this work is to determine in a contactless optical manner the thickness of the wetting layer as a function of indium content starting with very low content and low-strain structures (at the limit of self-assembled dot creation) up to typical InAs dots on gallium arsenide with the effective wetting layer as thin as 1.5 ML. The observed optical transitions have been identified based on energy level calculations for thin rectangular wells and using effective mass approximation in an envelope function approach in which the well width has been treated as a fitting parameter to the- experimental data.