We describe a method for the study of electron trajectories which depends on the electron impact excitation of neutral argon atoms to metastable states. The metastable atom probe (MAP) allows us to determine the current density in an electron beam as a function of two arbitrary coordinates with a spatial resolution better than 0.5 mm. The probe is sensitive to the effects of all factors which influence the electron trajectories, yet the MAP operates with such low atom beam densities that the perturbation introduced by the probe is very small indeed. The application of the MAP to the study of student-built electron guns is described; in particular an experimental verification of Snell's law for electron beams is presented.