With no record keeping like a log book or materials receipts, even a good archeologist isn't going to be able to decipher the TRUE maintenance history on those engines. A good mechanic will probably be able to give you a good "gut" report by looking at how things are in general in the engine room. He can do things like cut open oil filters and fuel filters to get a feel for how things are NOW and that COULD be an indicator of how things have been done in the past. Detroits are famous for being a bit "messy" with oil leaks and are actually designed to discharge oil from the blower air box and should be allowed to do so. Your mechanic will want to make sure the air box drain tubes are clear and air is flowing from them while the engines are running. If accessible, these engines do have inspection covers on the side of the block that can be removed and you can visually inspect the piston ring's condition. You can't always see everything that is going on because it is just a "window" into the cylinder. A broken ring will sometimes go undetected but you can definitely see how the rings are wearing through there.
You can have a compression test done but be warned that, on most large displacement diesel engines, this is an expensive proposition. Each injector has to be removed, one at a time, and the engine operated on the remaining cylinders while compression is taken in that cylinder. So, it is very labor intensive and takes a GOOD and experienced tech a long time to perform in order to get accurate results.
One alternative for well equipped servicemen is to use an oscilloscope and do what is called a "relative" compression test. The starter current draw is monitored for ramping up and down while cranking the engine. The tool is programmed to assign firing order for the engine and the "current ramps" or spikes associated with each cylinder are then compared to one another. Not an exact test by any means but can be used to isolate a failing cylinder rather quickly.
One thing you could do to protect yourself is to remove an oil sample and send it for analysis. But, that still only gives you what is going on NOW inside that engine. For example, if the oil was just changed, it will probably test as new oil with no contaminants unless you have a holed cooler or a bearing is rapidly failing.
Another test would be for a closed cooling system getting combustion gasses from a leaking head gasket and for the system's ability to hold pressure. Can't be done on keel cooled boats though.
Here's a tale to ponder:
I once sold a 1962 Chevy Impala to a friend. I was really proud of that car and the way it ran. I took really great care of the engine and transmission. I gave my friend a "smokin" deal on the car and I was ABSOLUTELY confident that he would be a happy new Chevy owner. It was a GREAT car!
He called me a week later to ask me how I could have done what I did to a friend. I was confused. "What happened?" I asked. He said that while on a trip to the big city, he stopped and filled up with gas. He got about 5 miles up the road and THE GAS TANK FELL OUT OF THE CAR WHILE GOING 70 MILES AN HOUR!
What??!! I was flabbergasted! How? Well, the car was old, we lived in Illinois and the roads are salted. The tank straps rusted through and I had never really looked at them before.
There was no fire. No one got injured. I offered to fix the car for him. "That won't be necessary" he said. "Why not" I asked. "I junked the car" he said. "Oh no man! I could have fixed that for you". "Well" he explained, "when we flagged a ride into town to try and find a way to get the car fixed, someone came along and pretty much removed as much on the engine and radiator and stuff under the hood that they could get and the car couldn't be repaired."
Moral of the tale? I didn't mean to screw him selling him a used car. Neither of us looked the car over carefully enough to detect the rotten gas tank straps. He declined my offer to give him his money back realizing that it really wasn't my fault that the car was stripped on the side of the highway. But, he was always a bit distant after that. He learned the meaning of the Latin phrase "caveat emptor"...let the buyer beware. I learned not to sell used cars to my family or friends.
As far as my ol' pappy's saying, I don't know what it means either. He just said it. He never explained it