I think there was a PBS documentary about him and Clinton a while ago? One of the people commenting on his casino bankruptcy said he didn't seem to be familiar with basic bookkeeping.
He talks about trade deficits with other countries in terms of winning and losing, without seeming to be aware that trade surpluses and deficits can each be good and bad depending on what you are buying and selling.
Trump claims he predicted that the housing bubble was about to explode, but he entered the mortgage selling market just before the 2008 crash.
Again, he refused to repay a loan to Deutsche Bank on the basis that the crash was like an act of God.
There is no evidence that he is good at anything except selling a brand.