1. Open or create a Markdown document (or another file type you want to enable spell-checking for)
  2. Go to Preferences > Settings - More > Syntax Specific - User
  3. Add the following code:

    {
        "spell_check": true
    }
    

If you’ve made any spelling errors or used some funky words, you should notice some of the titular red squiggly lines.

Incidentally, you can set any option for individual syntaxes in this manner.