Some engine hour meters don't give an accurate reading because they are installed incorrectly. i.e. activated when the start circuit is on. I have seen a number of boats with thousands of hours on them.....and there is no way that could occur.
The best way is to do the detective work....copies of old bills, log books
, appearance of engine.....sometimes a nicely "painted engine" just looks outta place.
One guy I know (my mentor in the biz) used to call the "Detroit Diesel
Tuneups" referring to the Alpine Green Paint