At CAS, UEFA called an expert on financial investigations as a witness to give his conclusions and to counter City's own expert on financial investigations.
In the PL case, we don't know but I would imagine they will present someone from the investigation team, at least, to explain what they did and how they found what they think they found. City would, presumably like to counter that with either cross-examination, or by presenting their own experts' opinions.
Maybe Nick Harris can deliver one or two of his whistle-blowers from the club. Although, as we think they are probably
@Prestwich_Blue and
@slbsn , probably be better to forget that :)