Unfortunately...even a great mechanic
dosent have x-ray vision...and he is giving you his best educated guess..My engine tested out fine during sea trial with him aboard with one or two minor electrical
deficiencies...after our first home sail I noticed the belts were not lined up like they use to be..once the belt tension was relieved I found the crank bolt holding the mail pulley on had completely backed out and the pulley fell off in my hands..
I know this is a good mechanic
I dealt with him personally for over a couple weeks and he seemed very knowledgeable and professional and came highly recommended..but he could not see it and there was no indication of what was about to happen... if that pulley would have come off at full RPM
it could have went through the bottom of the hull
...fortunately nothing was damaged and I installed a new seal and reinstalled to proper tork with a new bolt.
My criteria would be
1) does it start easily ( means good compression
2) does it smoke ( should not have any blue some white ok while cold but should go away) There should not be excessive black smoke under load either.
3) any visible oil
or coolant leaks
( black dirty messy engine )
4) stays in temperature ( run it hard and find this out ) I would rather have deficiencies any of the above over this condition )
( outside and inside ) inside is hard to gage but a good indication is to pull a hose or two look at the goose necks especially on the heat exchanger
..how bad are they eaten away.. if it has internal zincs pull them as well and peer into all the cavities you have opened as best you can.
6) Pull an oil
sample if you have doubts about maintenance ( but with out a prior bench mark or if they just changed the oil...it may be meaningless )
A good mechanic can get fancy but the above will do 90% of your diagnosis for you..and unless you know Supper mans phone
number you wont find out much more..
Bottom line for me...If it runs solid it more then likely is solid.......untill it isn't.