First of all you need to create a JavaScript project (seeCreating a JavaScript Project page). Also, see below to know how to enable it in non-project files.

Type checking is done by Flow. To permit Flow to type check your files in the project, you must add the// @flow comment (see how it works:Flow - Docs and.flowconfig). To see what is the error, go with the cursor over the text. It will appear a red box with the error, like this:

You could also copy to clipboard the error.

Errors will be shown also at the bottom of the editor:

Also, you will get errors about wrong .flowconfig file configuration:

If you want disable Flow type checking completely on your project, just go to theproject_settings.json and set"flow_checker_enabled": false.

To be able to apply the Flow type checking also in a file that is not an element of a project, you could activate the option inTools > JavaScript Enhancements > Use Flow checker on current view (Not used in project):

You can navigate errors in a project using:

or opening the Sublime Text Command Palette and typingFlow::