It can be interpret by examining the kind of words used in the text. 

sometimes the kind of the word is totally different from its real meaning, the situation must be clear in which a sentence is said, and another important thing is the exclamatory words, hopefully that might make a sense to the tones!!

Hard to give a more specific answer without more detail, but this kind of problem would generally be considered a kind of "sentiment analysis" in the NLP literature.  That search term should give you some good starting points.

If I were you, I'd look at LIWC. It may be able to give you the info you're looking for.