A good question.
One way would be to hold your hand on the bypass tube (item 14 in the parts page link) as the engine runs and begins to heat up. If the thermostat is operating correctly, there will be a point when, as the thermostat begins to open, the temperature of that tube will suddenly change.
A better method, that would yield more useful info, would be to use an infrared temp gun aimed at the cylinder head where the thermostat is located. The head should rapidly get hotter and hotter and then the temperature will go down a bit and back up a bit in what is called "modulation" as the thermostat opens and closes to maintain a constant operating temp. You would expect this "stabilization" to take place at around 160f to 170f if all is well. If it is a bit higher or lower than that, it's nothing to be concerned about. Heat transfer to the surface of the metal for the gun to read will vary greatly depending on several factors including what the water make up is.
The important thing here is that the head temperature goes up pretty fast from a dead cold start and then stabilizes as the stat begins to work. If the head temp goes up slow and gradual and never reaches a point over about 145f to 150f then your thermostat is most likely stuck partial open and the engine won't burn it's fuel properly.
Of course, if the temp goes significantly OVER 170f then you may have a stat that is not opening fully and you may experience overheat as the engine is worked harder.
http://www.boats.net/parts/search/H.../BF75 LA/INLET MANIFOLD THERMOSTAT/parts.html
As far as fuel pumps causing more fuel contamination than thermostats, MY experience has been just the opposite. I have seen more "making of oil" due to faulty stats than due to leaking pumps. It's why I mentioned it on this thread.
Typically a leaking pump will cause the oil to rise rapidly after an oil change as opposed to a stuck stat causing a very gradual rise between oil changes.
Neither is good for the engine and each should be eliminated as a possibility before further operation.
Good luck.