Short answer
Run the engine to full throttle in clear water and note the maximum rpm it reaches. If it cannot reach the manufacturer's rated maximum, the propeller is overloading it. Overload shows as black smoke, higher temperatures, poor fuel economy and accelerated wear, and is corrected by propeller pitch, hull cleaning or both.
You will need: Tachometer or engine display, clean hull, open water
This is the most useful single measurement an owner can take, and almost nobody takes it.
The test
In clear water, with a reasonably clean hull and normal load, open the throttle fully and note the maximum rpm the engine will hold. Compare it with the rated maximum in your manual.
- Reaches or slightly exceeds rated rpm: the propeller match is broadly right.
- Falls short: the engine is overloaded. It is being asked for more torque than it can make at that speed, and everything downstream of that suffers.
Do the test the same way each time — same load, same water, same hull condition — so the numbers are comparable.
What overload actually costs
Black smoke, elevated exhaust and coolant temperatures, higher fuel consumption for the same speed, carbon build-up, and shortened engine life. Many "overheating" and "black smoke" faults are simply overload wearing a different hat — see what causes black smoke.
Fixing it
Clean the hull and propeller first: fouling alone can account for the shortfall, especially in warm Australian water. If the engine still cannot reach rated rpm on a clean bottom, the propeller needs re-pitching or replacing, which is a job for a propeller shop working from your engine, gearbox ratio and hull data. See marine propellers explained.
Never "fix" overload by adjusting fuel delivery. That buries the symptom and accelerates the damage.