Just apply the Lorentz's force $\vec{F}=q\vec{v}\times\vec{B}$ to your simulation.
For B field in z direction, velocity in x-y plane.
$F_x=q*v_y*B_z$
$F_y=-q*v_x*B_z$
or the differential equations are
$\frac{d^2v_x}{dt}=\frac{q}{m}v_y*Bz$
$\frac{d^2v_y}{dt}=-\frac{q}{m}v_x*Bz$

And you can check out whether $v=\sqrt{v_x*v_x+v_y*v_y}$ is a constant or not.