Short answer
Log fuel added against hours run and the rpm you held, across several trips, and divide. Alternatively run a measured passage at a steady rpm and refill to the same point. Published fuel curves are a starting point only — your hull, propeller, fouling and load decide the real figure.
You will need: Logbook, hour meter, known refuelling point, or a flow meter
Two identical engines in different boats burn quite different amounts of fuel. The only number worth planning with is the one measured on your boat.
Two methods
The logbook method. Record litres added, hours run and the rpm you typically held, over several refuels. Divide litres by hours for a working average. It is coarse but it reflects real use.
The measured-passage method. Fill to a repeatable point, run a known distance at a steady rpm, refill to the same point. This gives litres per hour and litres per nautical mile at that rpm, which is more useful for planning.
A flow meter or engine data on a multifunction display shortens all of this considerably and lets you see the effect of rpm changes live — see engine instruments and alarms.
Turning it into range
Range is usable fuel divided by litres per hour, multiplied by the speed you hold. Usable fuel is not tank capacity: subtract what the pickup cannot reach, and hold a reserve — commonly a fifth or more — for weather, tide and diversion. Full method: fuel consumption and range.
Re-measure after changes
Antifouling, a propeller change, a repower or a significant load change all move the number, usually more than owners expect. So does hull fouling through a warm season.