We explore scanning probe microscopy (SPM) to demonstrate local measurement of motion in an isolated, metal-coated silicon nanomechanical resonator actuated by a harmonic photothermal source. In the measurement, a plasmonic nanofocusing element is integrated with a SPM probe for efficient concentration of propagating surface plasmon polaritons at the apex of probe tip and confinement of light in the gap between the probe-tip and surface of the nanomechanical resonator. Upon illuminating the nanomechanical resonator with an intensity modulated laser, the light source is partially absorbed leading to heating, thermal expansion in the metal coating and the silicon substrate, and a heat-induced bending moment in the resonator. The bending motion of the resonator changes the width of the gap between the probe-tip and the resonator leading to modulation of the scattered light intensity in the far-field. We explore a heterodyne demodulation approach to suppress unwanted background scattering and resolve the mode shapes of the first and second bending modes of the resonator. The measurement technique allows for an all-optical actuation and detection of mechanical vibrations in micro- and nanostructures with sub-wavelength lateral spatial resolution. Furthermore, the measurement technique enables the actuation of nanomechanical resonators over a broad frequency range, and measurement of their steady state displacement with high frequency resolution and signal-to-noise ratio.