Discrediting evidence is often a defendant's best line of defense.
In general, three types of evidence will typically be offered at trial: testimonial evidence (statements of witnesses on the stand); physical evidence (such as a murder weapon or a charred item from an arson); and demonstrative evidence (such as a chart showing steps in an embezzlement conspiracy). Each type of evidence is subject to a credibility challenge.
At trial, the judge reviews the offered evidence in light of evidentiary rules designed to weed out untrustworthy or irrelevant evidence and evidence obtained illegally. Juries then make credibility determinations on the evidence presented to them. Once a judge lets a piece of evidence in, it's part of the attorneys' job to convince jurors of the credibility or lack of credibility of that evidence.
In virtually every trial, much of the evidence gathered by each side before trial is never presented to the jury. This happens because each side has made motions to suppress (keep out) the evidence before trial and the judge has ruled in favor of certain of those motions. Motions to suppress that are based on legal defects in the gathering of the evidence are among the type that the judge would decide. For example, a defendant who claimed that police mislabeled or sloppily handled physical evidence might be able to convince the judge that, because of the risk of mistake, the evidence should be excluded. Similarly, judges rule on claims that certain evidence is "hearsay" (see discussion below).
The term credibility is used in criminal law just as it is used in daily speech: it means worthy of belief. Some types of evidence are considered, on their face, to lack credibility. Hearsay is the most common example of evidence that on its face lacks credibility.
Hearsay is a statement that is made out of court and then offered in court to prove the truth of the matter asserted in the statement (basically, he said she said). For example, the in-court statement of a witness that another witness told her that "he saw the defendant assault someone" is a hearsay statement if the prosecution offers it to prove that the defendant was the assailant. Hearsay is considered suspect because it is "second hand" and not subject to cross-examine by the other party. A judge will bar hearsay evidence for that reason. Where a witness testifies to her own observation of an assault (non-hearsay), the defendant's attorney has the opportunity to cross-examine her observation and test its credibility. Then it is up to the jurors to decide if they are persuaded that the evidence is credible.
But, not all hearsay is excluded; some hearsay makes it in. Numerous exceptions exist that allow a judge to admit hearsay evidence, including "dying declarations." A "dying declaration" is a statement made by an individual when death is imminent. Such a statement is considered more trustworthy than most hearsay statements because of the circumstances under which it is made (under the theory that a person has less motivation to lie when at death's door). The other hearsay exceptions similarly involve circumstances under which statements are considered more likely to be credible
Two other types of credibility challenges that are usually decided by the judge are constitutional and procedural challenges based on the way the police gathered and handled evidence. A common constitutional challenge is a Miranda challenge. Statements that were taken by police without advising a suspect of his Miranda rights are generally barred. The Miranda "right to remain silent" and right to have an attorney are rights aimed, in part, at preventing authorities from coercing confessions from people in custody. A coerced confession necessarily lacks credibility because it is not freely given and something other than the truth (such as intimidation) may be motivating it.
Common procedural challenges include evidence tampering, where a defendant's attorney argues that the state has offered altered evidence, and "chain-of-custody" violations, where a defendant's attorney argues that the police failed to maintain proper control over evidence after it was taken, such that it may have been compromised. An example of the latter may be where police cannot account for the whereabouts of a knife smeared with blood during some period between its collection at the crime scene and presentation at trial.
If the prosecution can establish a good chain of custody for that blood-smeared knife and the judge admits it into evidence, the jury will likely hear or see DNA, fingerprint, or other forensics evidence regarding who touched the knife and whose blood is on it. Often, this evidence is presented through expert witnesses who specialize in such analyses. The credibility of even expert witnesses will be challenged by the opposing party, such as by questioning the expert on the dollar amount of fees the hiring party paid the expert. And, the opposing party will call an expert to challenge the other side's experts, including their expertise the conclusions they drew based on the analysis of the forensic evidence. Then, the jurors will huddle and choose a winner in the "battle of the experts," often by deciding which of the experts was most credible.
Bolstering the credibility of the defendant's evidence and tearing down the credibility of the state's evidence is one of the most important jobs of a criminal defense attorney. The truth matters, but so does the way it is presented. An experienced trial lawyer knows how to present a strong case.