How can I test what my readme.md file will look like before committing to github?

Visual Studio Code has the option to edit and preview md file changes. Since VS Code is platform independent, this would work for Windows, Mac and Linux.

To switch between views, press Ctrl+Shift+V in the editor. You can view the preview side-by-side (Ctrl+K V) with the file you are editing and see changes reflected in real-time as you edit.

Also...

Q: Does VS Code support GitHub Flavored Markdown?

A: No, VS Code targets the CommonMark Markdown specification using the markdown-it library. GitHub is moving toward the CommonMark specification.

The preview button is this:

More details here

Atom works nicely out of the box - just open the Markdown file and hit Ctrl+Shift+M to toggle the Markdown preview panel next to it. It handles HTML and images also.

This is a pretty old question, however since I stumbled upon it while searching the internet maybe my answer is useful to others. I just found a very useful CLI tool for rendering GitHub flavored markdown: grip. It uses GitHub's API, thus renders quite well.

Frankly, the developer of grip, has a more elaborate answer on these very similar questions:

• Is there a command line utility for rendering github flavored markdown?
• What's the best way to edit GitHub's Readme.md?

Many ways: If you're on a Mac, use Mou.

If you want to test in a browser, you could try StackEdit, as pointed out by @Aaron or Dillinger since Notepag seems to be down now. Personally I use Dillinger since it just works and saves all my documents in my browser's local database.