Maybe the algorithms are secret...
That doesn't mean to say that the inputs used in a specific case are. Presumably the inputs used are recorded by the court.
If the defending side wanted to argue fairness they should be entitled to call for a re-run of the algorithm but with each contentious input changed by request (I'm thinking a similar idea to that used for jury selection). If changing a date of birth, for example, had a significant impact on the outcome then you know that there is something untoward embedded in there.
Using that as an example, let's say that the authors of the system singled out their exact dates of births for special treatment (should their algorithm end up analysing them) then nobody would be any the wiser, except hitting on one at random.
The legal system should dictate that the defense is allowed to make x attempts to game radically different results with inputs within certain limits of those used against them in order to undermine the validity of the algorithm used.
One important point to note is that the algorithms used should be frozen for the length of the trial, otherwise a rerun, or amendments made to inputs during the trial may produce different results due to the addition of further field data.