If you mean the BeePre 300B Pre-Amp, then the output is from the two sets of jacks at the back right - XLR's and RCA jacks in the standard configuration. The three sets of RCA jacks inboard are the inputs.
So, left to right: Input 3, Input 2, Input 1, Output 1 (RCA) Output 2 (XLR)
They should be labelled with Right and Left Inputs and Outputs.
Why would they be? Up to the builder whether they are labelled or not.