Highlighted is a small part of a complicated high energy
neutrino event produced in the Fermilab 15-foot bubble chamber
filled with a neon hydrogen mixture.
A positron (red) emerging from an electron-positron pair, produced by a gamma ray, curves round through about 180 degrees. Then it seems to change charge: it begins to curve in the opposite direction (blue).
What has happened is that the positron has run head-on into a (more-or-less from the point of view of particle physics) stationary electron - transferring all its momentum.
This tells us that the mass of the positron equals the mass of the electron.
A qualitative appreciation of this can be obtained by considering the head-on collision of one snooker ball with another: professionals are able to bring the `projectile' ball to a halt. (Discuss what would happen if the projectile were made of (a) polystyrene, and (b) lead.)
Further details can be found in A Lot can happen in a few Billionths of a Second and A Simple Estimate of the Mass of a Positron.