I don't think you can diagnose over-engineering after the fact. Unless you were in the room, or have access to written specs from a meticulously documented team

I don't think you can diagnose over-engineering after the fact. Unless you were in the room, or have access to written specs from a meticulously documented team, you don't understand the conditions under which the code was written. Can you prove it in a court of law? No. But you still can often easily diagnose it. And you'll usually be correct. For example the conditions under which https://wiki.c2.com/?SecondSystemEffect https://wiki.c2.com/?SecondSystemEffect happens are well-known, and the pattern has been repeated often in the half-century since it was named. If it is a second system, and you can find complicating abstractions in it that get used exactly once in practice, then with better than 90% odds it was over-engineered. And the farther you dig, the stronger the evidence will become.