You would look in a narrator POV or an omnipotent POV and then determine how the author has been biased towards one side by looking at adjectives, statements, and other information.
Here's an example of a biased statement:
Donald Trump rudely stated in his horrible speech yesterday that all "foreigners should get out of the country".
Here's an example of a unbiased statement:
Donald Trump stated in his speech yesterday that all "foreigners should get out of the country", thus speculating many riots and protests.
By looking an omnipotent perspective, you are able to see the subjective statements the Author has made. This is also indicative of the Author's context
(Sorry if my answer was not up to standards, it's my first one! :) )
