Hi Syed! Here are two uses: 1. SERIOUS: never would have thought something to be the case. (eg. He was the culprit all along? I wouldn't have guessed!) 2 SARCASTIC: . knew it all the time because it was so obvious. (eg. Samantha "Miss Pretty" Evans is the prom queen? I wouldn't have guessed!) Source: http://idioms.thefreedictionary.com/never+would+have+guessed)

wouldn't have known