There is a good deal of contention and confusion over what constitutes real evidence of Unidentified Flying Objects. Much of what is accepted has to do with the group or organization that is looking at the data, and what they think is valid proof.
Let's not mistake the fact that any evidence, from personal observation to physical evidence, is real UFO evidence. However the type of evidence and how it was acquired is what gives one piece of data more weight than another.
We can look at the kinds of data that have been gathered, to get a better idea about this.
By far, the majority of UFO data is from direct eyewitness observation. This actually defines the general meaning of UFO. If I see something that is flying and cannot say what it is, it is a UFO by definition. Obviously, this isn't what most people think of when they think about UFOs. After all, if I am not a trained observer and can't tell one aircraft from another, my observation won't mean much, from a serious research standpoint.
This brings the importance of credibility to the forefront. Many sightings have been reported and documented by police officers, military personnel, pilots, clergy, and others who would be considered very credible witnesses. Interestingly, many of these reports are so well detailed that they would easily be admissible in a court of law if the topic were murder instead of UFO observation.
Photographic and electronic evidence is also part of the data. A large number of pictures, including motion pictures, have been taken of UFO phenomenon. Sound and radar recordings have also been produced. There is still an issue of credibility with this sort of data, however, since such data can and often has been faked. It usually takes a good deal of careful expert analysis to determine if the data is real.
There is also physical data involved. Though not as common, some items have been gathered which defy our technology to produce or reproduce. Some supposed landing sites have been shown to contain radioactivity at higher levels than they should. People, plants, and animals have occasionally shown various reactions or traumas after having a UFO encounter. Each of these is physical evidence.
The gathering of the data must also be considered when weighing how valid the evidence is. Just as with a crime scene, the data can become tainted if poor collection methods are used. It also needs to be understood that faked data, though not valid, is still evidence and it should be considered as such until it is proven to be invalid.
In each case, there is an indication that the encounter or sighting really happened, and that the evidence exists that supports the belief that the occurrence is real. How strong that evidence is, though, again depends on who is viewing it and the kind of evidence or combination of kinds of evidence, that is involved.