Coincidentally, the wise birds at Cambridge University Music Library have put together a great posting about ways to date rare old sheet music, whilst Karen here has been discussing RISM numbers with friends at both RISM and Copac. (RISM numbers are another excellent way of identifying early music - and the part of RISM dealing with early printed music is now freely available online. Meanwhile, Copac is the big national combined catalogue for UK university and national libraries. Librarians call it a 'union catalogue'.)
Keep abreast of the latest thinking - why not take a look at these links?