The response of vertically oriented multi-walled nanotubes (MWNTs) to fluid flows forms a basis for monitoring shear forces at the nanoscale. We report on the modulation of polarized light transmission through a MWNT mat, as a function of the nanotube axis relative to the laser polarization. Nanotubes are deflected under fluid flow, with fluid pressures corresponding to pico-Newton range forces. This deflection is measured as a function of transmitted laser light intensity. While the response of the CNTs to the flows could be modeled through standard elastic beam deflection theory, their recovery, after the force removal, invokes viscoelastic behavior due to CNT disentanglement. The fluid flow direction, as well as its pressure, can also be determined by monitoring the change in transmitted laser polarization.