The Directory of U.S. Labor Organizations (BNA Books) tells you each union's officers, function, office locations, etc. The Institute of Industrial Relations posts links to union Web sites in the U.S. and in other countries.
The DOL's Office of Labor-Management Standards lets you search for Annual Financial Reports (LM-2, LM-3 and LM-4) filed by labor unions since 2000 through UnionReports.gov. These includes information on the union's net assets, investments, charitable contributions, salaries, etc. You can also search the data to find unions or union employees meeting specified criteria.
Union constitutions, by-laws and "Labor Organization Information Reports" (LM-1s) and older annual reports are not available online, but you can order copies from the DOL. For faster service, BNAPlus (800-372-1033) sells constitutions and by-laws. To learn about a union's retirement plan, get the Form 5500 and other information returns filed with the Department of Labor using the databases listed in the "Retirement Plans" entry.
To see if a company has a union, and/or which union, you can contact the company directly or contact the likely union for that area. Alternatively, you can hire BNA Plus to search for you (703-341-3272). They search a variety of databases, including their CBA database and labor-oriented news, to give you about the best answer you can get from published sources.
For statistical information, the Bureau of Labor Statistics compiles an annual Economic News Release called "Union Membership" that covers the number and percentage of union members in the workforce (in total, by age, race industry, etc.) and what they earn. In addition, BNAPlus publishes a semi-annual report called NLRB Representation and Decertification Election Statistics and the Union Membership and Earnings Data Book.
To serve process on a union, see Service of Process.