Wolf Messing and van Hannussen happen to be two of my "heroes" and I can assure you that both were but weren't frauds
Much of what they employed has been a well known yet well kept secret in occult circles for generations, so on that point alone there is a legitimacy to what these gents used in order to fulfill the public's idea of what a Psychic/Mind Reader was. Van Hannussen however, was an entertainer and rather established as such along with his role of being confidant and personal Reader for most of the key players in the NAZI Party during it's rise to power. Memory serving me correctly, he was murdered on the Kristallnacht not just because he was a Jew but because he was a Psychic, which was something that had created problems for Hitler (long, complicated story).
There's a great deal of documentation around van Hannussen and some of the stunts he got away with when it came to the German belief in things paranormal/occult.
Messing was more of a Post War character and though he did "perform", it was more akin to demonstration vs. theatrical muse. His top fan being Joseph Stalin who looked at him much in the same way we saw the nobleman in the film "The Illusionist" viewing the mage... Messing was a curiosity for Stalin, who saw himself as being intelligent enough to explain how Messing accomplished his various demonstrations.
In either case we are looking at individuals that used the same general techniques for a similar set of reasons; if one were found to be using trickery or deception in those days (withing the Germanic nations) prison was inevitable and death quite likely. For this reason these two and others, were inspired to learn specific skills that were known to the Roma people for "centuries" (according to legend and somewhat sustained by history). Ironically a gent by the name of Randall Brown had already become a sensation in the United States using the very same method; something he'd learnt as a lad. While Brown knew brief success, those that he shared the method with who were more natural showmen, became major headliners; they knew how to exploit the method and make it much bigger than it really was.
Now in skimming over this thread I couldn't help but chuckle to myself because of the many theories tossed about, and while some do mirror the more minor ingredients tossed into the broth either man employed, they are far from what was really going on. I will however give you a subtle nudge in the right direction -- Dowsing.
