Mavericks: Enable virtual keyboard on login screen
This answer focuses on the case where you want to access the virtual keyboard because of layout/external keyboard uncertainties.

If you only have access to a mouse/pad this answer will not help you.



There is a keyboard shortcut that propose you to access to Accessibility Options, and it is working at login screen. (Only on High Sierra.)

Opt-Cmd-F5

And for MacbookPro w/ Touch Bar:

fn-Opt-Cmd-F5 or Triple click on TouchID button

From this new window, you can click and enable the virtual keyboard:

Here is the Mac accessibility shortcuts ref from Apple.

EDIT 2: for macOS Mojave

If the problem is that you're not sure about your keyboard layout, try this:

Opt-Enter : It lets you enter the login in a clear text field. Use it to visually check your layout \o/

EDIT: (added information from comments)


Only available on High Sierra
Shortcut for MacbookPro with Touch Bar