Developers constantly learn new APIs, but often lack necessary information from documentation, resorting instead to popular question-and-answer platforms such as Stack Overflow. In this paper, we investigate how to use recent machine-learning based knowledge extraction techniques to automatically identify pairs of comparable API methods and the sentences describing the comparison from Stack Overflow answers. We first built a prototype that can be stocked with a dataset of comparable API methods and provides tool-tips to users in search results and in API documentation. We conducted a user study with this tool based on a dataset of TensorFlow comparable API methods spanning 198 hand-annotated facts from Stack Overflow posts. This study confirmed that providing comparable API methods is useful in API learning: developers using our tool were significantly more aware of the comparable API methods and better understood the differences between them. We then created SOREL, an comparable API methods knowledge extraction tool trained on our hand-annotated corpus, which achieves a 71% precision and 55% recall at discovering our manually extracted facts and discovers 433 pairs of comparable API methods from thousands of unseen SO posts. This work highlights the merit of jointly studying programming assistance tools and constructing machine learning techniques to power them.