Other than the low oil pressure indication inside the tachometer, there is no way to know for sure .
One member here added a mechanical, not an electrical oil pressure indicator, but it's something you're going to have to 'engineer' yourself, there are no kits available .
The few set-ups I've seen, have a 'manifold' threaded into the oil pressure switch hole, then either a direct reading mechanical indicator, threaded into it, or a pressure sending sensor added, plus the associated wiring and indicators .