Open access journals will usually state that they are open access on their website. This is usually located in their “About this journal” or “About us” section. An open access journal will usually state what you can do with their articles. Here is an example: http://www.academicleadership.org/about/ Articles that are available in PubMed are not necessarily open access. However, some articles in PubMed Central (PMC) are. PMC articles will be noted with an icon in PubMed. This can get tricky because some articles are temporarily open access due to new regulations required for NIH funded research.
Here are some places that you can find open access journals:
Directory of Open Access Journals: http://www.doaj.org/
PubMed Central: http://www.ncbi.nlm.nih.gov/pmc/journals/?filter=t1
BioMed Central: http://www.biomedcentral.com/about