For a visual indicator get a hand sprayer with water and a tiny drop of fairy liquid and spray away liberally, you will see it bubble up at the point it is leaking. Bare in mind it could be leaking in more than one area so Mark the leak with a sharpie and keep going until you are happy you have no more leaks.

I normally have a pressure gauge on both cylinders, if your centre seal has gone the you will see the pressure rising on the opposite cylinder to the one you are pressurising. Although I recently had an engine with a brand new centre bearing do this, the bearing wasnt quite seating in the cases so I had to dress the bearing seat to cure it but I would say that would be pretty rare.

Favourite places for leaks are the main seals, carb insulator and the r/h cylinder base gasket where the crank case halves mate.

The latest engine I rebuilt was a real head scratcher, pressure was dropping but no sign of a leak. Ended up dumping it in a bucket of water (only do this if you are going to strip it again, it gets everywhere) and it was bubbling out of the left cylinder water outlet. The crank case had some corrosion pitting on the left hand cylinder mating surface and was leaking from the case into the cylinder water jacket! That f****r really tried my patience!