Unfortunately no “Other Party Or Group” has the resources or standing to investigate vote fraud.
For instance most judges would toss out a Libertarian investigation into vote fraud because they have no standing - they weren’t going to win anyway so they weren’t harmed.
This is assuming, of course, the Libertarians had the resources for such an investigation in the first place.
I find that hard to believe. Only "official" parties have standing?
That certainly suggests several avenues of attack. As a group, honest legal voters far outnumber all other groups combined!
And they are certainly the most clearly injured.