You might see some turbulence in the tank, the return is below the oil level to avoid foaming. If you don't have any oil circulating it will soon become very obvious.
Pop off the fuel line(s) and attach some tube to the petcock. See how fast the fuel flows in ON and RES. You'll probably see a big difference; there's a bit more pressure on RES normally (because of the raised pickup for ON) but there should be plentiful flow both ways. The normal difference gets greater as the tank empties, they should be close to identical with a full tank.
You might have some blockage in the actual petcock or in the ON pickup tube. Some petcocks are held together with screws and are easy to service. Others are riveted together, you can drill out the rivets then drill and tap the petcock body for reassembly.
The petcock is either attached with a double-threaded nut or screwed on from inside the tank. Either way you drain the tank and remove it, there's a filter "sock" over the tank side of the petcock works that's probably turned to mush and can be discarded, you will need an inline filter in the fuel line to keep crud out of the carbs if it's gone... and you should use them anyway. The brass tube sticking up is the ON pickup, it doesn't usually get plugged but that can happen. The RES pickup at the bottom and the metal around there do get corroded as any water in the tank collects at the lowest point... right around the RES pickup.
It's a good idea (IMO) to run on RES for the first few miles after a fillup to drain any water that might have collected.
