How can Norwegian Customs know who repaired their boats abroad?

I read in this article as well as this and this (all in Norwegian, so use Google Translate), that Norwegian Customs control private boats (owned by individuals) coming from abroad, to see if they repaired their boats abroad. The reason for that is that people are supposed to pay tax to Norway for the repair they performed on the boats abroad.

How can the Norwegian Customs ascertain whether a boat was repaired abroad? Let us say that I have a leisure both in terrible condition. I sail my boat down to Poland, where I repair it into tip-top shape for much cheaper than what the cost would have been in Norway. I then sail back to Norway, and upon returning to Norway, customs stop my boat. They can obviously see that my boat is in tip-top condition, but how can they know the boat was in bad condition when i left Norway? How can they know it was not in tip-top condition all along, or repaired in Norway before I left? I may have repaired it myself, or kept it continually maintained, for all they know.