Very often it can be determined from the nature of the report itself.

If you say, "I saw with the naked eye a featureless, point source amber light of approximately -1 magnitude traveling at a uniform angular velocity in a straight line from roughly 030 degrees to about 270 degrees at about 10 degrees/second, reaching a maximum elevation above the horizon of about 30 degrees over the horizon. There was no sound, and it was visible for about 5-10 seconds.", it carries much more weight than "I saw a bright yellow light, about a mile away, bigger than any airplane, going from right to left".

This is an extreme case, of course, but the report which gives numerical
information with some clue as to its precision, and which does not interject details which would be impossible to determine by any observer, you tend to have more confidence in the report. The first observer may be deliberately perpetrating a hoax, but at least he isn't likely to be misidentifying a common object.

By identifying himself and allowing himself to be cross-examined, and by providing some biographical information (amateur astronomer, airline pilot, naval lookout, etc.), it gives the examiner of the report some idea of which kinds of questions need o be asked. In my experience, when an unqualified observer sees an unusual object or event, even if he is scrupulously honest about what he relates, he has a tendency to embellish the observation each time he recounts it.

This is not a criticism, if I witnessed a bank holdup I doubt if I'd be able to ID the perps in a line-up or remember to memorize their getaway car's licence plate. I never had to do that before, or given it much thought, and I'd probably be too rattled to be a good witness. I feel confident I would be an excellent UFO witness, though, because of my training, experience, and interests.