The evidence is his sister.
Maryanne Trump Barry has long been a federal judge. But not long ago, there was a charge of judicial misconduct made against her, which came as a result of a major New York Times article that went into great detail about tax fraud the Trump family used in connection with the inheritance from their father. As a result of that charge, an internal judicial investigation was opened. And 10 days later she resigned. Which ended the investigation.
(The charge made wasn't for a federal crime, but dealt with legal ethics. That's why, with her no longer being a judge, there was no reason to investigate her.)
Either that is wildly coincidental and she just happened to resign from a lifetime appointment to the federal bench for no known reasons, or It appears pretty clear that she resigned because she didn't want an investigation into family business over the inheritance, which seemingly dealt with tax fraud. And given that it was likely all a part of the same scheme -- since it not being part of the same fraudulent scheme makes no sense -- then her brother was involved with it all, as well.
As I said, any fraud over the family inheritance is most-probably beyond the statute of limitations. Though I don't know if that fraud has had to be continually perpetuated through the years to the present day. At the very least, people who commit such major fraud for such huge amounts are...well, fraudulent, and what we know publicly about his dealings shows it was only beginning.
What we also now know is that -- remarkably amazing coincidences aside -- that original inheritance fraud was massive enough to get a federal judge to retire to shut down an internal investigation.
Here's the thing --
The SDNY probably noticed. And if they didn't start investigating it themselves after the New York Times report, they probably are now...