- As of yet Unknown
As of yet, no known Signamancer has been introduced, but apparently Transylvito has some.
While no actual Signamancers or Signamancy has appeared in Erfworld canon yet, we can infer some things about the discipline's nature from indirect comments on it. Rob Balder has said about Signamancy, "Signamancy. Like the Peace sign, man. Like the protest sign, man. Signs, signs, everywhere a sign. Far out." The precise meaning of this is unclear; however, he is drawing a direct link between a visible symbol or word and a desired result. This is further strengthened by the nature of Signamantic philosophy - that all physical appearances are simply symbolic manifestations of the underlying "truth" of Erfworld.Erf-b1.5-p037 Based on these, it seems that Signamancy is in some way concerned with the relationship between "real" things and the symbols that represent them - the relationship between the map and the territory, so to speak. Signamancy is also a Hippiemancy, which may indicate certain things about the flavor and details of the discipline, but since no other Hippiemancy is adequately described as of now, it is hard to draw concrete conclusions from this connection.
As Signamancy supposedly deals with the relation of what is seen and what is there it's likely that Signamancers are at least good at psychiatry if it's not actually included in the abilities.
Real World References
Also, analogous to Enlightenment/Awakening in general  particularly in light of Signamancy being an discipline of Hippiemancy with a sister discipline of Flower Power.