Witnesses in both criminal and civil cases can’t testify about something they don’t personally know about. The “personal knowledge” rule (Federal Rule of Evidence 602) requires that all non-expert witnesses testify based on firsthand information. It’s not enough that they got the information secondhand (as is the case with hearsay), even from a reliable source. Nor can they testify about information they have good reason to believe, but can only assume is true.
For example, suppose Monty Vesuvius is going to testify that a defendant charged with drunk driving swilled down four martinis before jumping behind the wheel of a car. Suppose Vesuvius says something like:
Each statement shows that Vesuvius lacks personal knowledge such that he cannot testify to the issue of what the defendant drank. However, Vesuvius can testify that he saw the defendant consume four drinks from glasses that martinis are usually served in—the prosecution can use that circumstantial evidence to argue that Vesuvius did, in fact, consume four martinis.
Usually, very little is required to show that a witness has the requisite personal knowledge to testify about an event. Most often, the judge will tilt toward letting questionable evidence in and leave it to the lawyers to bring out in cross-examination or closing argument just how shaky the evidence is.
Example: Babe Bear is charged with trespassing onto property owned by Goldie Locks; Bear’s defense is mistaken identity. At trial, the prosecution wants Locks to identify Bear as the individual who ran out of her door when she came home after visiting her grandmother. In an evidence minitrial, Bear offers evidence that Locks had been drinking wine and was inebriated when she came home. Bear’s evidence also shows that Locks, who normally wears glasses, was not wearing them when she came home. Locks may make her identification, because she is testifying from personal knowledge—that is, she claims to have personally observed the events to which she is testifying. Whether the problems that Bear pointed out mean that Locks’s testimony is not believable is a separate question—dealing with the weight of the evidence—that the judge or jury will have to decide at the end of the case.
Example: Sue Emmall is charged with possession of heroin. The prosecution plans to call Hy Enlow, Sue’s next-door neighbor, to testify that he saw Sue holding a baggie of white powder that he believed to be cocaine. Hy can testify from personal knowledge that Sue was holding a baggie containing white powder. However, only an expert drug analyst could testify to whether the powder was actually cocaine.