These days most package documentation is provided as a PDF file and, if it is installed on your system, it can usually be obtained using the texdoc application. If you have a terminal or command prompt, you can access it by typing texdoc followed by the name of the package. For example, to obtain the documentation for the datetime package run:
texdoc datetimeSometimes this may produce just the documented code rather than the user manual. For example:
texdoc flowframwill display the documented code. However, in this instance, the first paragraph of that document tells you that the user manual is in ffuserguide.pdf in which case
texdoc ffuserguidewill produce the user manual.
In some cases (especially for older packages) the documentation may be contained in a README file in the documentation directory or it may be embedded as comments either at the start or the end of the .sty or .cls file.
Alternatively, if the documentation was not installed on your system, you can obtain it from CTAN. You can either use the search box on the CTAN home page or you can use the URL http://ctan.org/pkg/name where name is the name of the package. For example, to obtain information on the glossaries package, you can use the URL http://ctan.org/pkg/glossaries and it will provide links to the documentation for that package.
There is also the TeXdoc Online Resource which is a dedicated server for querying and browsing TeX and LaTeX package information and documentation, where you can search for packages or topics.