Whether a journal is open access is not a yes or no question and, instead, openness exists along a continuum. PLOS, SPARC, and the Open Access Scholarly Publishers Association have worked together to develop a guide that assists authors to "understand the components that define Open Access journals, learn what makes a journal more open vs. less open, [and] make informed decisions about where to publish." The HowOpenIsIt? guide is available in multiple languages and can be downloaded from the SPARC website.
The Directory of Open Access Journals is another resource for authors looking to publish in an open access journal and for researchers who would like to locate open access publications: "The Directory of Open Access Journals is a service that indexes high quality, peer reviewed Open Access research journals, periodicals and their articles' metadata. The Directory aims to be comprehensive and cover all open access academic journals that use an appropriate quality control system (see below for definitions) and is not limited to particular languages or subject areas. The Directory aims to increase the visibility and ease of use of open access academic journals—regardless of size and country of origin—thereby promoting their visibility, usage and impact."