Springer's Open Access Glossary defines Gold Open Access as "publication of an article by Open Access publishers or in typically peer-reviewed Open Access journals and financed either upfront through publishing fees or through public or private funds."
For more information, check out Peter Suber's webpage about gold open access.
Published in a journal that isn't Gold Open Access? Check Sherpa/RoMEO for your publisher's Green Open Access (self-archiving) policy:
A predatory publisher exploits the gold open access model in order to gain a profit from authors' fees. In addition, predatory publishers frequently do not adhere to academic standards, such as peer review, during the publication process. Many are not transparent about their operations and use official sounding names in order to recruit authors.
Jeffery Beall, Scholarly Initiatives Librarian at the University of Colorado Denver, has compiled lists of publishers and individual journals that should be treated with caution. A Q&A explaining his criteria can be found in the Chronicle of Higher Education.
If you are unsure about the quality of an open access journal in your discipline, please contact your subject liaison.