Use WhoIs lookup anytime you want to perform a search to see who owns a domain name, how many pages from a site are listed with Google or even search WhoIs address listings for a website's owner.
The word act, as used in the term Act of Congress, is a common, not a proper noun. The capitalization of the word act is deprecated by some dictionaries and usage authorities. Some writers, in particular the U.S. Code, capitalize Act.
If it refers to a governmental body that uses it as part of its name, such as Federal Trade Commission, it would be capitalized. Regarding the tax return, if you are writing the name of a specific tax return form, it is considered a proper noun and would be capitalized. Form 706 United States Estate Tax Return.