It is well known that living cells sense and respond to their surrounding mechanical microenvironment. Cells have been shown to sense and respond to mechanical stimuli such as topography, rigidity, applied force, and pressure. What remains to be understood is how cells respond to mechanical stimuli on short, fast time scales. I study how cells sense their local mechanical microenvironment on substrates of varying stiffnesses. Using a technique I developed that incorporates fluorescent nanoparticles in a single top layer in hydrogels, I precisely track cell-induced motion of the substrate with high resolution imaging. Characterizing nanoscale cell fluctuations could provide insight as to how cells deterministically perform large-scale activities such as spreading, differentiation, and migration. Such insight could potentially help characterize other largely unknown phenomena, like the time evolution of disease.