The velocity distributions of 250nm diameter gold nanospheres trapped in the evanescent fields of optical waveguides are studied. The automated analysis of a large number of particles and temporal frames is described. It is used to show that the envelope of the particles' speed follows the mode intensity profile of the evanescent field along a length of the waveguide and across its width. Modal beating in a dual-moded waveguide is mapped by analysis of nanoparticle distributions above the waveguide. A modal power of _150mW at l=1066nm in a Cs+ ion-exchanged monomode waveguide results in speeds of up to 500microm/s.