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