Q. How can I tell whether something is a primary or a secondary source?

See the description provided by Princeton.  

Examples of primary sources include:

• Diary of Anne Frank - Experiences of a Jewish family during WWII 
• The Constitution of Canada - Canadian History 
• A journal article reporting NEW research or findings 
• Weavings and pottery - Native American history 
• Plato's Republic - Women in Ancient Greece 

Examples of secondary sources include:

• A journal/magazine article which interprets or reviews previous findings 
• A history textbook 
• A book about the effects of WWI