The closed part of the cooling system should have a pressurized radiator cap. Get the engine warmed up and see if there is any pressure. If there is you know there is no leak in the heat exchanger. However, if there is no pressure it could also just be a bad radiator cap. Caps need replacement about every five years. I’ve seen them go ten. Another thing to try is getting a hydrometer for testing the concentration of antifreeze. Something less than 50% would allow corrosion even if the coolant has been changed every five years to replenish the rust inhibitors. Back a little more than five years ago, antifreeze seemed to last about two years for the inhibitors and scale formation in radiators was also a big issue. Also, if there are chlorides (found in table salt and seawater) corrosion will be accelerated. The most vulnerable parts of the engine for corrosion are the soft plugs on the side of the block and water pump impeller, but with a little luck, you are probably OK, especially if the soft plugs are bronze. I do not know anything about heat exchangers; however, my guess is that since the metals are designed to resist seawater corrosion on one side, the side for the engine is also probably quite corrosion resistant. Don’t forget to check your thermostat. If open just sitting there on the bench, replace it. If you really want to know, check to see if it starts to open at the rated temperature stamped on the thermostat. I did a search and found this Diesel operating temperature
a posting by HELLOSAILOR, see post number 5 that looks right. I would qualify his answer by saying that a high temperature thermostat in a direct cooled engine in fresh water will result in scale in the engine if the water has a lot of hardness. I saw this on boats in Lake Mead on the Colorado River where scale causing lots of head cracks.