Guess how you tell a lot of these things?
By DRIVING THE CAR. Which the mechanic may do.
I can assure you, no mechanic takes apart a cv axle, takes measurements, and determines if they are out of spec with a micrometer. "Oh, your cv axle has 32.6% of its life left according to specs". Like no. Just no.
What is this weird idea people have that mechanics are magic? They have special tricks that tell them things?
No, you pay them $200, they put the car on the hoist, wiggle shit and visually inspect things I don't give a crap about. They can't magically tell you the engine is worn unless it's compression is way fucked, or it is clearly burning oil.
At a shop rate of $145/hour, a proper full on inspection that was worth anything would be a full days work or about $1k.
How does a mechanic tell you a transmission is screwed or severely worn? Unless it's already acting fucky, which a TEST DRIVE would tell you, they won't be able to give you anymore info.
Killramos sounds like one of those people that is afraid to take an engine over 4k rpm. "Oh no, somebody bagged my car for 5 seconds before I squealed like a bitch they were hurting it". Really?