I would think that before an aerial sighting could be declared possibly 'extraterrestrial' in nature, it would have to satisfy several criterion.
1. There should not be a terrestrial craft of similar shape, size, construction or characteristics in existence (at the time of the sighting).
2. It should exhibit flight characteristics that are known to be not possible by terrestrial craft, manned or unmanned. These characteristics should
be distinguishable from optical effects, such as camera movement. (iow, you move a camera focused on a light and the light tracings seem to fly around
in impossible loops), or optical illusion.
3. It should be clearly visible as a 'craft' and not an optical illusion.
4. It should be sighted within 500 feet of the observer or closer (the classic Hynek CE-I event)
5. There should be at least two reliable witnesses, interviewed separately that give a consistent account. Ideally there should be photographic or
filmed evidence. The witnesses should have more than an ordinary interest in ET, UFOs, or paranormal subjects; ideally they should be skeptics.
Ideally the event would be a classic CE-II, in which there are ground traces, or the craft seen on the ground, photographed and traces recorded
Anything that is further away than 500 feet, or consisting solely of lights in the sky or distant daylight discs would be concluded as at best, simply
In addition the sighting of such craft that may be deemed as 'non-terrestrial' would not necessarily be associated with aliens until proven
otherwise - ideally by visualization of occupants.
Bear in mind that with current technology it would be possible, with sufficient funding to fake all of this, even to a live observer. So even though
these characteristics might be seen, it would still not be classified as 'proof positive'.
(obviously there are other things to look out for, but this is a start)
Hope this is of interest.
[edit on 1-3-2007 by Badge01]