1. How Can I tell where a given Periodical is currently being indexed, or once was indexed, and what issues have been indexed? (Indexes frequently do not cover complete runs of a given periodical, but start indexing at some point after the periodical begins publishing, and stop indexing before the periodical stops publishing.)
Ulrichsweb This online directory tells where current and recently ceased publications have been indexed. It covers the entire world, and currently lists over 125,000 currently published periodicals in every country of the world. But over the years, it has covered over 300,000 periodicals, most of which have ceased publication. (Do they have a database of all 300,000+ periodicals?)
https://ulrichsweb.serialssolutions.com/title/1387678616925/244120
Ulrich's International Directory, hard copy, tells where a give periodical was indexed. These old hardcopy directories are often found in large academic and public libraries. This hard copy directory began in 1932 with its first edition, and so there has been double coverage of periodicals by Ulrich's and Ayer's (Gale) from 1932 through the present. But in the years when the Standard Periodical Directory and Periodicals Directory were being published, there was triple and quadruple coverage for some years.
Ayer's Periodical Directory (This was being published under different titles from 1880 through 1986, the 118th edition, after which Gale took it over (see below). It began in 1880 with the title N. W. Ayer & son's Directory of newspapers and periodicals, annual, 1880-1909; Merged with Rowell's directory in 1910, and continued Rowell's numbering, so the 1910 issue was the 42nd edition. 42nd (1910) - 103rd (1971) year. Ayer directory of publications, annual, 1972-1983. IMS-Ayer directory of publications, annual, 115th ed. (1983) - 118th ed. (1986). Issues 1880 through 1909 are available full text at: http://lcweb2.loc.gov/diglib/vols/loc.gdc.sr.sn91012091/default.html
Gale Directory of Publications and Broadcast Media: This directory took over the Ayer's periodical directory after the last Ayer Directory in 1986 (118th edition), and continued the numbering system. The first issue was in 1987 (the 119th edition) under the title, Gale Directory of Publications and Broadcast Media.
Periodicals Directory (only published a few editions)
Rowell's American Newspaper Directory: This directory began in 1869 and contained a description of all the newspapers and periodicals published in the United States and territories, Dominion of Canada and Newfoundland, and of the towns and cities in which they are published, together with a statement or estimate of the average number of copies... 1st (1869) - 40th (1908) year. In 1910 it merged with Ayer's and Ayer's continued Rowell's numbering system. So there was coverage by two periodical directories from 1880 through 1909.
Standard Periodical Directory: This periodical directory was begun in 1964 by Oxbridge Communications, Inc., New York, NY, and continued to be published through , which was the last issue published.
Willing's Press Guide: This was a comprehensive index and handbook of the press of the United Kingdom of Great Britain, Northern Ireland and the Irish Republic : together with the principal Commonwealth publications. It began in 1874, published in London by James Willing, and covered periodicals and newspapers. And since it covered the Commonwealth, it essentially covered most of the periodicals in the world being published in English outside the United States. The 122nd edition was 1996.
Willing's European Press Guide: This began in 1966 and continues to be published. It is a companion to Willing's Press Guide, and covers newspapers and periodicals of Europe outside the Commonwealth.
2. How can I tell what new periodicals have started up in a give year?: To do this, just go to Ulrichsweb and search or sort for all periodicals begun in the latest year. This way you will be able to continually be updated. You could actually download the entire list with "full" data, and see all the subjects, etc.