Interesting piece at motorsport.com:
In some cases, teams have been able to reconvene stewards after events to look at cases when a new element has emerged.
But according to the FIA's International Sporting Code, this avenue has been closed off because of time limits. A right of review needs to be brought 14 calendar days after the publication of the final classification of a competitor, and no less than four days prior to the FIA prize-giving ceremony.
One of the reasons for the FIA electing to reach a settlement over the Ferrari matter is that it was likely unable to prove beyond doubt that the team had broken the regulations, while the outfit itself could not show that it had definitely complied with the rules at all time.
In such a stalemate scenario, the options were therefore either to reach an agreement to move on, or take it further and go to court.
However, article 2.7 of Formula 1's technical regulations states that: "It is the duty of each competitor to satisfy the FIA technical delegate and the stewards that his automobile complies with these regulations in their entirety at all times during an Event."
The FIA statutes do offer the chance for the governing body to open a disciplinary inquiry if there are suspicions that the rules have been broken. The matter could then be submitted to the FIA International Tribunal.
One avenue for teams could be to ask the FIA to pursue this route, especially if they have more detailed information regarding the Ferrari power unit than the governing body has been made aware of up until now.
I don't know exactly how the teams would legally force the FIA to reveal details in the confines of the FIA's regulation. I don't think that is possible without the FIA's help (and since it is in effect a case against the FIA, I don't think the FIA would be helping).
However, there is still civil court.