Your first resource will be https://kangax.github.io/compat-table/, which provides very thorough and complete tables showing, feature by feature, what is supported on JS engines everywhere. Depending on your specific needs, you might be able to totally dispense with transpiling, but it's certain you should be careful before taking such a measure!

The following screenshot shows Kangax at work:

A second web tool that you should be aware of is Can I use... at https://caniuse.com/. In this site, you can search for any feature (be it JS, HTML, or CSS), and you'll get to see what browser versions support it or not. A comment is relevant: this site only provides information for desktop and mobile browsers; you cannot see if a feature is supported in Node, for example. The following screenshot shows Can I use... at work: