I don't have a Master, but it would normally be the smaller of the two.
If the engine runs, you will be able to see the fuel flow through the clear pipe.

 

Small cup (200ml) under a connection - disconnect one at a time - switch on ignition (do not start) - from that you can see which one brings fuel to engine for a few seconds

 

Black pipe is return

 

Excellent. Thanks for the help 😀