Are state marriage records publicly accessible online? Is there a database (free or subscription-only) where such information can be found?
I believe they're public record throughout the United States, and available online for most of the United States.
I don't know of any single, national repository for marriage records, so I'd recommend Googling around (e.g., "California marriage records" or "San Diego marriage records"). If you're looking for records from a jurisdiction (or time) that are not available online, you could also try using some of the free online genealogy resources, which make those records available. The LDS Church has a particularly powerful service, which, as I recall, will allow you to search for a person and even pull up a scanned copy of licenses that they have available.