The reliable sources are those which have some authenticity. The authenticity is based on data and its analysis or some research or some experiments.
The science text books are based on some reference books / journals and are published if authored by an authentic person (with experience and knowledge)
government internet sites are based on facts / analysis so are considered reliable, generally
reference journal are most reliable as those are internationally reviewed and published from a well repute publisher.
However weekly news magazines are just source of information and may have a different view or content. So these can be considered to be least reliable.