The county or other tax assessor places a value on a property for taxes. That can be negotiated, but usually isn't. In any case, the government and property owner agree that's the value for taxes. The value of a property otherwise is a guess unless you sell it. There is no sure way to know what a property is really worth any other way.
For example, I sold two houses about two years ago, one for a bit more than $400K, another for just shy of $500K. Both went to a company that flips houses nationally. The first sold about 8 months later for a bit more than I sold it for. The second sold about two months ago for about $425 K a big loss for the company. Property values are what somebody is willing to pay for it and not a fixed thing.
Unless Trump grossly-- made the values many times what they were possibly worth and misstated the condition and stuff on the property--the prosecution's got nothing. Now, if he did, and he lied about buildings, structures, etc., as well maybe the prosecutor has something, but I doubt it given she's another Trump hater.