SETI, the Search for ExtraTerrestrial Intelligence, is a private group who is trying to scientifically find evidence of other life in the universe. They use radio telescopes to look for signals that might be sent from extraterrestrials. When SETI detects a signal they think could be from some sort of alien intelligence, they first try to falsify their results. They check if it could have been caused by some radio emitter on the earth, or some known astronomical object. They have other scientists check their results by looking to see if they find the same signal. In their own FAQ, SETI explains this perfectly.
In the past, there were several unexplained and intriguing signals detected in SETI experiments. Perhaps the most famous of these was the "Wow" signal picked up at the Ohio State Radio Observatory in 1977. However, none of these signals was ever detected again, and for scientists that's not good enough to claim success and boogie off to Stockholm to collect a Nobel Prize. You wouldn't believe cold fusion unless researchers other than the discoverers could duplicate it in their labs. The same is true of extraterrestrial signals: they are credible only when they can be found more than once.The other side of the coin is the UFOlogists. While some sites try to be scientific, they miss some key components. First, the whole premise of their argument is flawed. They are taking photos and anecdotal reports of unknown objects, and using those as evidence. I wrote just recently why this doesn't work. Briefly, if you have an unknown it is simply that, an unknown, not evidence. UFOlogists also will ignore alternative explanations that contradict their interpretation of the evidence. For example, they will ignore the idea that a video or photo could have been faked. Instead, they make unsupported statements like, "They can't all be fakes." There is also the tendency to portray scientists as "dogmatic." To anyone who works in science or with scientists, this claim is ridiculous. Scientists love new ideas that challenge what we think is true. Those ideas, however, must meet the standard of evidence.
In his Cosmos series, Carl Sagan made a great point in regard to UFOs. He said, "What counts is not what sounds plausible, not what we'd like to believe, not what one or two witnesses claim, but only what is supported by hard evidence, rigorously and skeptically examined." We all want there to be a easy cure for all illness, but that doesn't make it real.
When you encounter a claim ask which model the claimant is following. Many cases are not going to be straightforward, but their are some clues you can look our for. Is the claimant putting their evidence out to scientists to examine, or makes excuses as to why scientists don't support them. Do they try and prove their own claim wrong, or do they ignore the arguments made against them. I hope that one day (preferably within my lifetime) we find life somewhere beyond our little planet. This is a proposition that excites all of us. It is interesting how people can take such different approaches to solving the same problem. Science is the tried and tested method for exploring the world, whether or not it gives us the answer we want.