Colloidal particles are used to characterize microscopic potential landscapes, which are defined on a sample surface and arise in ensembles of particles. The positions of the particles are recorded using video microscopy. Analysis of the positions, which the particles occupy during their Brownian motion, yields the exact shape of the surface potential, in which the particles move. The underlying principle of our measurements is well-known from measurements using total internal reflection microscopy; in contrast to these measurements, our scheme can be expanded to measurements of inter-particle interactions. As an example, we demonstrate the measurement of interactions between two magnetic particles, sedimenting towards a potential barrier in a tilted geometry.