As many have pointed out, the geometry of the Seneca fuel tanks make a visual inspection prior to flight practically impossible if the aircraft is less than full.
However, the aircraft is fitted with fuel flow indicators and from memory at about 24/24 and leaned, the fuel flow will be about 10 USG per side making a total of 20USG per hour consumption.
Thus unlike the C172 or C150 or similar types, the pilot has a direct readout of the actual fuel consumption as the flight progresses.
Not having enough fuel to divert is absolutely no problem - Shoreham has several independent runways. However, that does not remove the 45 min final reserve.
As to accurately knowing how much fuel is in the aircraft before departure - Weight is the answer. The aircraft has a certified empty weight. If simple electronic scales are available at the aerodrome then simply weighing the aircraft without people or baggage will reveal how much fuel is on board and 6 pounds per US Galon is a nice round figure to keep things simple.
How about putting a weight platform on front of the pumps?
Regards,
DFC