I have an oil pressure gauge that replaces the oil pressure switch for testing purposes.
Obviously its located down by the oil filler and so not visible whilst riding - ok, you could fit an elbow and have a good look down.
I got it off ebay but originaly it came from 'evoluzione cyclesports'
part no: 50102
The gauge looks pretty much like any generic 7 kg/cm2 or 100 lb/in2 fluid pressure gauge.
Some one like 'Pirtek' should have them fairly cheaply.
You could tee off the standard pressure switch and look down if the light came on.
Crust