How does a PC detect a VGA monitor?
Computers detect screens by something called Display Data Channel

It's a low rate data signal on four pins in the VGA connector that tells the computer about its presence, allowed modes and frequencies, etc.

You can still use a screen without DDC, but you need some pull-up or pull-down resistors to tell the computer there's a screen at all. You'll also have to force the adapter to produce the signal, and you'll have to set up the monitor parameters manually.