Project Hessdalen is one group dedicated to researching these particular phenomena. What they have found so far indicates an earthly explanation.
Whether or not we can tell if a UFO report is a plasma or not is a little more complicated. Thus the need for as much information and detail for every account as possible to make a determination if it is or isn't plasma. That is, if plasma is a possibility to begin with.
Sorry for going OT a bit there zoser, I'll let you get back to your discussion of whatever case you deem appropriate. I just wanted to let it be known that if something unexplainable in the sky isn't ours it doesn't by default mean it is theirs.