As @dmckee suggested in his comment, the leading edge and more specialized fields often haven't fully standardized on a preferred notation. My experience with that is that individual authors and/or journals will summarize their notation in a table at the beginning or end of a text book or journal article. For more established notation, I have found the following.
Quoting from the ACS Style Guide (3rd Ed), chapter 13 "Conventions in Chemistry," p 255:
Detailed recommendations from the International Union of Pure and Applied Chemistry (IUPAC, http://www.iupac.org) are given in the book titled Quantities, Units and Symbols in Physical Chemistry, 2nd Edition, nicknamed the "green book," published by Blackwell Science, Oxford, U.K., 1993. Updates are published as articles in the journal Pure and Applied Chemistry.
Detailed recommendations from the International Organization for Standardization (ISO, http://www.iso.org) are given in the ISO Standards Handbook 2, Quantities and Units, published by ISO, Geneva, Switzerland, 1993. Some individual standards have been amended, and their updates are available at http://www.iso.org/iso/en/progs-service/ISOstore/store.html. the National Institute of Standards (NIST) Special Publication 330, 2001 Edition, available at http://physics.nist.gov/Pubs/SP330/sp330.pdf, is the U.S. updated edition of the English version of the Bureau International des Poids et Mesures.
All of those links work, though none of them are directly useful. The IUPAC website eventually led me to their Gold Book, which contains this table of recommended symbols for physical quantities. Appendix 13-1 of the ACS Style Guide is called "Symbols for Commonly Used Physical Quantities," and contains several tables, organized by field, of common physics and chemistry quantities. In addition, the most recent edition of the CRC Handbook that I own is the 86th (2004-2005). Section 2 of that edition is titled "Symbols, Terminology, and Nomenclature." In addition to the tables of symbols and abbreviations, it also contains a table of the Hebrew, Greek, and Russian alphabets, and several pages of definitions of common scientific terms. The CRC Handbook also cites the IPUAC Red Book and the ISO Standards book. It also cites a book called Symbols, Units, Nomenclature, and Fundamental Constants in Physics, Document IUPAP-25, 1987; also published in Physica, 146A 1-68, 1987 (items 3-9 on the linked page, but note that Elsevier will want $31.50 each for those seven items).