Question: I was recently party to a preliminary hearing in a criminal case in which a 911 call was played. The content of the 911 call was very beneficial to the defendant. During the hearing, the judge said something to the effect of, "It would be a question for the jury whether the caller was 'being honest.'"

In other words, the judge was suggesting that the caller to the 911 was being disengenuous and deliberately conveying a false impression -- something that I think would never occur to most people because the call seemed completely genuine. Also, the caller would have had no motive to faking their mental state. I got the idea that the judge was looking for some angle to discredit the 911 call out of interest to see the defendant convicted.

In any case, from the judge's remark I got the idea that he might say something like that during the trial, or give the jury an instruction designed to make them think the 911 caller was putting on an elaborate act.

Is a judge allowed to do this?

Answer: 
It is the jury's job to evaluate the credibility of the witnesses, and it is the judge's job to inform them of that responsbility.

It is not appropriate, however, for the judge to indicate to the jury what answer they should come to on those questions.

In [*Quercia v. United States*, 289 U.S. 466 (1933)](https://supreme.justia.com/cases/federal/us/289/466/), the defendant in a drug case took the stand to deny the charges. Before the jury went to deliberate, the judge made the following observation:

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> I am going to tell you what I think of the defendant's testimony. You may have noticed, Mr. Foreman and gentlemen, that he wiped his hands during his testimony. It is rather a curious thing, but that is almost always an indication of lying. Why it should be so we don't know, but that is the fact. I think that every single word that man said, except when he agreed with the Government's testimony, was a lie.
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The jury convicted, but the U.S. Supreme Court reversed, holding that the instruction was an error. It said that the judge has the right, generally speaking, to comment on the evidence, but that right is not unlimited, because juries are likely to be swayed by the judge's assessments, even if he instructs them to make their own decisions:

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> The influence of the trial judge on the jury is necessarily and properly of great weight and his lightest word or intimation is received with deference, and may prove controlling. This court has accordingly emphasized the duty of the trial judge to use great care that an expression of opinion upon the evidence should be so given as not to mislead, and especially that it should not be one-sided; that deductions and theories not warranted by the evidence should be studiously avoided.
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The comment you seem to be imagining is a closer call than this, but I think most judges would agree it would be inappropriate.

At a preliminary hearing, though, where there is no jury, there is no real problem with the judge making that comment. If I were the defense attorney, I'd be glad he did, as it would help inform my decision about whether to pursue a jury trial or a bench trial.
Of course he can, but in some jurisdictions he will have to choose his words carefully.

Depending on tone or pace of voice, "it's for the jury to decide (this or that)" will often be heard by the jury to mean "but I don't believe it".

Since the meaning depends on tone or pace of voice, who could ever prove anything against the judge?