
When viewing with the crystal at the bottom, the right side is (-) down, the left side is (+) down. Look in the bottom of the battery pockets...the right one has a bevel(chamfer) to stop the square corner of a positive from going down al the way. Unfortunately, the bevels are not shaped right to prevent "cell reversal damage" in cases where the contacts stick up like on this one, but they are instructive.
Also, right abouve the crystal there are + and - marks, which correspond to my suggestion.