Imagine that you are sitting in a chamber with your back to one wall. An electron beam,moving horizontally from the back wall towards the front wall,is deflected by a strong magnetic field to your right side. What is the direction of the magnetic field?

(DOWNWARD) The direction of the magnetic field is determined using Fleming's left-hand rule.
According to this rule,if the forefinger points in the direction of the magnetic field,the middle finger in the direction of the current,then the thumb points in the direction of the force (deflection).
$1$. The electron beam moves from the back wall to the front wall. Since electrons are negatively charged,the direction of the conventional current is from the front wall to the back wall.
$2$. The force (deflection) is directed towards your right side.
$3$. Applying Fleming's left-hand rule: Point your middle finger towards the back wall (direction of current) and your thumb towards your right (direction of force). Your forefinger will then point downwards.
Therefore,the direction of the magnetic field is downward.