A default font is the text styling that is usually used by a computer when a document is opened, such as Times New Roman.
Open the Sublime Text settings file: Mac OS X: Sublime Text 3 > Preferences > Settings - User. Windows: Preferences > Settings - User. Linux: Preferences > Settings - User.
Sublime Text may be downloaded and evaluated for free, however a license must be purchased for continued use.
Sublime Text is a shareware cross-platform source code editor. It natively supports many programming languages and markup languages. Users can expand its functionality with plugins, typically community-built and maintained under free-software licenses.
On Linux it's Monospace 10 pt. (the exact monospace font used may vary on different Linux distributions or versions), on Windows it's Consolas 10 pt., and on OS X it's Menlo Regular 12 pt.

(The color scheme is Neon, the syntax highlighting is from PackageDev, and the font is Liberation Mono
This information is found in the Packages/Default directory (where Packages is the directory opened by the Preferences → Browse Packages... menu option), in the Preferences (OS).sublime-settings file where OS is one of Windows, Linux, or OSX.
You should only customize the font (or any other setting) in Packages/User/Preferences.sublime-settings, opened by Preferences → Settings—User, as Settings—Default is over-written on upgrade, and also serves as a backup in case you really screw something up in your user settings. This is the case for both the main Sublime settings as well as those for extra packages/plugins.
These default fonts are the same in Sublime Text 2, Sublime Text 3, and the new version currently in development.
On my system (Windows 8.1), Sublime 2 shows default font "Consolas". You can find yours by following this procedure:
view.settings().get('font_face')
You will find your default font.
Yes. You can use Console of Sublime with (Linux):
Ctrl + `
And type:
view.settings().get('font_face')
Get any setting the same way.
To add to MattDMo's answer, you can get the exact font that's used on Linux like so (the example is from Xubuntu 14.04):
$ fc-match Monospace
DejaVuSansMono.ttf: "DejaVu Sans Mono" "Book"
The default font on windows 10 is Consolas
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