equery u CATEGORY/PKG will give you the USE flags for
CATEGORY/PKG, it will show the flags available (and to be used according to your USE settings) as well as the flags actually used for any packages already installed. Useful if you need to remember what was used for an installed package.
If you then run the command
euse -E FLAG or
euse -D FLAG you can enable or disable FLAG globally. As mentinoed in another comment
flaggie can adjust
/etc/portage/package.use, but you shouldn't need to touch that often; you would want to avoid adding something a hundred times that could just be set globally.