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.<PERSON>/the whole inheritance
Now I find all this evidence to be indicative and not absolute, unfortunately, especially if we assume that MR. <PERSON> held true to his claim that he cannot call for his alibi since she went on an atlantic cruise. (see the scene where <PERSON> visits Mr. <PERSON> 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 (<PERSON>) in court.
At the the shooting scene, when the local police captain (its late, forgive for not checking) <PERSON> arrives asking why they're are shooting, <PERSON> accuses Mr. <PERSON> and claims to have him cornered. Mr.<PERSON> counters by implicating him in the murders of <PERSON> and his sister, Attorney <PERSON> and Madame <PERSON> herself. 918
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 <PERSON>'s enforcer <PERSON> (played by <PERSON>) was at the prison looking for clues after Mr.<PERSON> escaped. <PERSON> met him at Mr.<PERSON> former cell and enquired what he might know about the death of Deputy <PERSON> whose corpse was found in an Art Museum missing four fingers. <PERSON> stern and almost cold-blooded reply left <PERSON> 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.<PERSON> was simply silent and interested in the proceedings while <PERSON> was nervously talking and gesturing. Why would he be ? If any one could be suspected of knowing the contents of the will it is <PERSON> IMO, although <PERSON><PERSON> could have been playing it.
Ultimately, at the trial, the Jury and the Judges seem quite entertained by Mr.<PERSON> account of the previous weeks' events. It is indicative but I think this scene was meant to make us aware that Mr.<PERSON>'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.<PERSON> 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. 918