The Right Hand rule is applied to find the induced current produced by the induced e.m.f . To know the induced e.m.f polarity , we need to know how current flow inside the 'battery'. The current flow from negative to positive inside the battery(the moving bar) . Further confirm by considering how current would flow outside the battery (the fixed bar), it flows out from the positive terminal to the negative terminal. With these two ways of confirming the polarity , you cannot be wrong.
The top end is positive and the bottom end is negative on the moving bar.
