Of course there were other signs on this job that it wasn't the work of pro. A more subtle signature of a knowledgeable plumber is the cleanliness of solder joints. The solder joints on this job looked like melted candles. You see pipe joints that look like old candlestick holders, you just know what you are in for. Don't get me wrong.
Pros make mistakes, too.
Their mistakes are usually not through lack of knowledge or workmanship but ones of omission or oversight. By the end of the day, things get rushed, or maybe the inspector shows up before you can check over your apprentice's work. These errors can add up to a failed inspection for even the most professional builder. They're what keep building inspectors gainfully employed. Back at the Cedar Street job, things are looking pretty good.