Generally most standard BIOS chips will do a "clone display" output if a VGA cable is plugged in. So I have no doubt it will work without needing to boot into the O/S. I used to do this all the time when I worked as a tech at a computer shop.

In the case your BIOS fails to do so then you need to hold down the "F" (function) key with the relative "F number" (ie F5) it will have a picture of a monitor on it.
This will force the BIOS to activate the dual video output.

The reason it should work straight away is because of this very scenario.
Manufactures were aware of the fact that if the screen fails on the laptop it renders it useless. Hence the majority of commercial standard laptops have accessible VGA output to allow the laptop to be useable in the case of a broken display.