How can I check if a string input is a word in the dictionary or not?

If I type "computer" it'll return true, if I type "cmputer" it will return false.

I've been looking at APIs like Wordnik: http://ift.tt/1DqYJZX

The problem is I'm not sure how to use it to check if a word is a part of the dictionary or not. Is there a better way to do this (maybe offline?)