Recall that the force of a magnetic field on a moving charged particle is

F = q v X B = q |v| |B| sin(phi),

where v is the velocity vector of the particle, B is the magnetic field vector, and phi is the angle between them. Thus if v is pointing up and B is pointing to the right, a positive particle will bend into the screen and a negative particle will bend out of the screen. Thus observing the direction of bending tells one the charge of the particle.