With the laptops i have used, simply having the external monitor plugged before you turn it on was enough to make it display the bios etc. After that its the OS that handles it.

Newer laptops i'm not very sure. Sometimes you can see a monitor logo in one of the keyboard keys, and use fn that to cycle internal/external/both (and some default to NOT use the fn key, whatever).

If you manage to get into the bios you might see a similar option as well.