How do I check if a business name is trademarked?

Search the USPTO's records for the name, and not only for the exact spelling: an examiner also refuses names that look, sound or mean alike, for goods that are related to yours. In the USPTO's free Trademark Search you type each spelling, sound-alike, plural and word split yourself, then filter by class and judge each record. LogoVerify runs those forms in one check (the same name, close spellings, common sound-alike spellings, plurals, split or joined words, each distinctive word), across live and pending marks in your goods and the related ones, and tells you which marks could block the name and why. Registering a company or a domain does not tell you whether a trademark already covers the name. It is a first screen, not legal advice.