This is rough sometimes. The first thing to do, of course, is to contact the organization that ran the conference or workshop you are interested in. If they can't help you, the best idea mentioned so far is to contact the Institute for Scientific Information, Inc. (ISI), and check with their Index to Scientific and Technical Proceedings (ISTP volumes).

      Institute for Scientific Information, Inc.
      3501 Market Street
      Philadelphia, PA 19104, USA
      Phone: +1.215.386.0100
      Fax: +1.215.386.6362
      Cable: SCINFO
      Telex: 84-5305