But how can you know which subject a database specializes in?

Most databases have an explanation at the first screen, if not, check the database "Help" function. Ask one of the librarians at the reference desk (yes, they WANT to help!) The University Library has many handouts that focus on specific subjects. These are called bibliographies and will suggest databases to use. The Libraries' Web page has a Subject Pages & Guides selection. Each connected subject page has listings of recommended database for that specific field of study.