Marked 2 is a great Mac utility for previewing Markdown files as fully rendered rich text. It works with many text and writing apps, including Bear.

With the macOS version of Bear you can preview a note in Marked 2 by selecting Note -> Preview in Marked in the menu bar. Notes do not have to be in Markdown compatibility mode to be previewed in Marked; Bear will take care of converting them.

As of Bear 1.7.15 and the Mac App Store version of Marked 2, you no longer need to authorize access to a .textbundle file in order to preview notes.