Then there's this from a Western Australian A/C install company...
I was told that the way an electrical inspector (do they even exist now?) could tell if an owner did their own electrical work was how neat and tidy it was done.
I have heard the exact same story in regards to a number of building processes. Everything from electrical, to framing, to .... you name it.
The professionals know how far they need to go for the result to pass inspection - by the relevant authority. The owner will want it "just so" and will take extra effort that a tradesman just can't afford.
A typical example I saw at Bunnings a few weeks ago... You have your tradie who will load up 30 lengths of timber and only pass up on one or two that are significantly bent or twisted. You then have a weekend warrior who will go through 50 pieces (or more) looking for 10, checking each and every one for straightness and defects, only going for the ones that look as perfect as possible. The tradie will be in and out in 10 minutes while the weekend warrior could spend an hour puttering around.
BTW - these are framing timbers I'm talking about. Not gonna see them when the wall sheeting is affixed.