The following scenes are key to answering this question (in chronological order):
- the deposition formalizing the accusations
- the scene at the 6th floor's cross-coat where both fronts shoot at each other
- the scene where they are opening the will
- the trial of Mr.Gustave/the whole inheritance
Now I find all this evidence to be indicative and not absolute, unfortunately, especially if we assume that MR. Gustave held true to his claim that he cannot call for his alibi since she went on an atlantic cruise. (see the scene where Zero visits Mr. Gustave in prison)
THe deposition is dated to the 12.October 32, 7 days before the murder. This might be a goof in the film but it is there. Such a fake deposition would be seriously incriminating for the heir apparent (Dmitri) in court.
At the the shooting scene, when the local police captain (its late, forgive for not checking) Henckels arrives asking why they're are shooting, Dmitri accuses Mr. Gustave and claims to have him cornered. Mr.Gustave counters by implicating him in the murders of Serge X and his sister, Attorney Kovacs and Madame D herself. These accusations wouldn't be taken seriously normally, but the case has been shrouded in mystery from the beginning. Key here is that when the Dmitri's enforcer Jopling (played by Willem Dafoe) was at the prison looking for clues after Mr.Gustave escaped. Henckels met him at Mr.Gustaves former cell and enquired what he might know about the death of Deputy Kovacs whose corpse was found in an Art Museum missing four fingers. Joplings stern and almost cold-blooded reply left Henckels wondering. Wouldn't you wonder when a guy treated you like that while wearing somewhat ornate brass knuckles ?
At the opening of the will Mr.Gustave was simply silent and interested in the proceedings while Dmitri was nervously talking and gesturing. Why would he be ? If any one could be suspected of knowing the contents of the will it is Dmitri IMO, although Mr.Gustave could have been playing it.
Ultimately, at the trial, the Jury and the Judges seem quite entertained by Mr.Gustave account of the previous weeks' events. It is indicative but I think this scene was meant to make us aware that Mr.Gustave's story is amusing or at least amusingly performed.
So, as I said, only circumstantial evidence as far as I can see. The deposition might be a goof while the trial could also be argued to incriminate Mr.Gustave in our eyes since he is beyond a doubt a very good storyteller and impressive persona whom one could imagine as a superb con-artist.