According to Langmuir, it has five characteristics. The effect under study is produced by a causative agent of barely detectable intensity. Furthermore, the size of the effect is essentially independent of the magnitude of the cause. Claims of great accuracy or sensitivity are made for the effect. The theories described are contrary to experience or known effects.
Any criticism is met by ad hoc excuses made up on the spur of the moment.
Any time you see a claim that meets some or all of these criteria, you should be suspicious. The effect may be real and exciting, but it should be met with some caution. This was around the time of the discovery of x rays. The idea was that N rays would pass through aluminum, but not iron. To detect the rays, an observer looks at a barely illuminated screen.