From this do you mean it would not be possible to determine if Trump is committing crimes by looking at his tax returns and the tax returns of his companies? If so this is badly wrong.
Just as an FYI the New York Times already uncovered strong evidence of criminal activity by Trump and his family using his dad's tax information.
The president has long sold himself as a self-made billionaire, but a Times investigation found that he received at least $413 million in today’s dollars from his father’s real estate empire, much of it through tax dodges in the 1990s.
www.nytimes.com
If given access to the requisite tax and business records I am highly confident that forensic accountants could determine if Trump was/is engaged in criminal activity, which he most likely was/is. I mean I guess there's always a chance he committed a bunch of crimes with his dad, never got punished for them, and decided to stop out of the goodness of his heart, but that is... unlikely.