How to check if sentiment analysis is required?
TextBlob, a Python package, does this (and much more). It uses a pre-trained model, thus requires no training. Given a sentence, TextBlob will return the polarity from -1 to 1. -1 is negative, 1 is positive, 0 is neutral. It will also return subjectivity from 0 (very objective) to 1 (very subjective).