One of the most efficient way to determine the mood of a piece of writing is to examine the word choices of the author. Many descriptive terms have a definite connotation, and by examining the additional meanings and looking for trends throughout the work we may begin to determine the mood.
For example, in Pickwick Papers by Dickens, the author writes the following:
“The river, reflecting the clear blue of the sky, glistened and sparkled as it flowed noiselessly on.”
The terms "clear blue", "glistened", "sparkled", and "noiselessly" convey a sense of calm beauty. Compare this to a description of waters that are "rough" or "roaring" and note that these latter violent descriptions would be used to convey a tense or potentially violent mood.
As another example, we may look to Wuthering Heights by Bronte:
“There was no moon, and everything beneath lay in misty darkness..."
With only a small number of words, the author effectively sets a mood of mystery. We are told twice over about the darkness of the scene, once directly and once by announcing the absence of moonlight. The description of mist help to further this mood by establishing a lack of vision and clarity.