There is a fair chance that the issue can be settled with DNA testing.
Word is it has been, that the results have been submitted to a journal.
The method of proof would be to compare claimed black decendents with known white decendents. But still it would be difficult. Fortunately, there is an ingenious alternative. Jefferson's Y-chromosome would not be diluted by the passage of generations. Then, assuming that among the decendents of Sally Hemming who may be alive today and may be decended from Jefferson, there are some who are patrilineal, they should also have the same Jeffersonian traits at loci on the Y-chromosome.