I've heard some writing advice to the effect that if you need a thesaurus to find a word, you shouldn't be using that word. There's a danger that the word won't fit the tone of the novel, or won't capture the exact meaning you want to convey.
I do use a thesaurus occasionally. Mainly I use it to generate alternatives for words or phrases I overuse. As I revise my novel, I've realized how many times my character's "stomach tightens" in a tense situation. Thanks to the emotion thesaurus over at The Bookshelf Muse, I've managed to think up some new ways to show how my character is feeling. Thanks, Angela (and Becca)!! My alternatives aren't always words taken straight from the thesaurus, but reading thesaurus entries does get the ideas flowing.
Some ways I check whether a word works:
1) I read my story (or that section of the story) out loud to hear how it sounds.
2) I ask myself, "Would my character use this word?"