Requirements differ per country. In most US job advertisements I notice that candidates preferably have completed both technical and librarian education. Try searching the web archive of PIUG discussion list using the terms "job posting" or "searcher position".
Required knowledge/skills are:
- basic patent law
- searching in online (patent) databases (you'll need several)
- and preferably also knowledge of the technical field you'll be searching
Training is mostly "on the job", under guidance of an experienced searcher. Most database producers offer training for their databases, this will teach you the technique of using their databases. It will not teach you how to analyze a search request and convert it into a proper search strategy, that is what you'll have to learn from a experienced searcher. We have an introduction training in the Netherlands <http://www.patentinformation.nl/> and I recently noticed some announcements by a group [in India] that they run a training program in this PIUG discussion-list.
To my knowledge there is no official licensing or certification yet, some attempts to come to a register for patent searchers are running, but it will take some time for this to come into practice.