There are certain clues which tend to indicate the presence of pseudoscientific methodology. These are not hard and fast rules - just because a theory fits some of the criteria below does not mean it should be discounted. Remember that many genuine scientific discoveries began life as the subject of ridicule so be fair when you pass judgment.
Having said that, if the clues below are featuring prominently when evaluating a theory or argument, it makes sense to be dubious.
What exactly is the nature of the evidence? Does the theory include a range of independent studies or does it rely on anecdotal evidence? The latter is a strong indicator of pseudoscience.
Any theory which has no support from empirical evidence must be considered highly suspect. Remember, experimental results and statistical data must be independently verifiable.
Any evidence should be directly relevant to the theory. Pseudoscience often looks for credibility by calling upon evidence which is legitimate, but not actually relevant.
Does the theory require you to discard any established laws of physics or other science? If so, is the new theory really more robust than the old?
Remember, the theory which is simplest and relies on the least number of unproven assumptions is most likely to be correct.
Does the theory hold up consistently, or does it only work intermittently? Can other researchers replicate the results?
Pseudoscientists often have excuses for why their theory is not consistent or will not work when applied by anyone else. This is a very serious flaw.
Does the theory include technical terms without explaining exactly what they mean? Many pseudoscientific theories talk about "vibrations" and "energy" without actually quantifying these things. Often the the terms are used incorrectly or out of context.
Terminology which has been invented specifically for the theory should be closely examined. Although such new terminology is often legitimate, it is often completely bogus.
Does the theory have documentation and supporting papers? Does this material include specific details and references? There really is no excuse for not documenting theories and experiments.
Does the theory have support from other researchers? If so, who are they and what are their credentials? What motivations might these people have for offering their support?
Is the theory or subject considered legitimate by recognised educational institutions? Is it a subject which is taught exclusively in non-credentialed institutions?
It is very easy to gain support from people who already believe a theory, or have some interest in seeing the theory advanced. It is more difficult to obtain support from skeptics and those who hold differing personal beliefs. However a valid theory will always attract at least some measure of support from the scientific community if the evidence holds up. It may take a while and be controversial, but it will happen. Theories such as quantum mechanics are examples of how even the most outrageous theory will gain support if the evidence is there. If a theory has absolutely no support from conventional science after several years of research, something is probably wrong with the theory.
A theory should not use historical beliefs, public acceptance or assumptions as support. Phrases to beware of include:
"Common sense" is a particularly dangerous thing. As often as not it actually means "common ignorance".
A lot of pseudoscience attempts to tap into the human sense of bewilderment at the universe. Emotive language such as the following, whilst being quite true, should not be considered a valid argument in support of a theory:
A valid theory will tend to make progress from year to year. New data will emerge from experiments and studies and more researchers will add to the body of documentation. If a theory has stagnated or failed to result in new data, there is probably a reason.
Ideally this should not be an issue since the scientific method ignores the proponent and concentrates on the theory itself. However, in the real world, we find that certain people and certain traits are synonymous with pseudoscience. Be careful to check the background of the proponent and be wary of habitual scammers. The following behaviours should be considered suspicious:
Also, be skeptical of qualifications and experience - it is common for these to be faked or exaggerated. For example, "I used to work for NASA" could easily mean "I held some minor position in an unrelated department for a short time before I was sacked for being a flake".