A particle is projected into a uniform magnetic field acting perpendicular to the plane of the paper. The field points into the paper. The trajectory shown could be that of a (see Fig.)

 

Considering the these directions

$\overrightarrow{v}$is along positive x-axis, and $\overrightarrow{B}$ is along positive y-axis $\Rightarrow \overrightarrow{v}x\overrightarrow{B}$is along positive z-axis ($\hat{i}x\hat{j}=\hat{k}$)

but force acting on the particle is $\overrightarrow{F}=q\left(\overrightarrow{v}x\overrightarrow{B}\right)$ is along negative z-axis. So we can conclude that the charge on the particle is negative.