How can I diagnose that my laptop lcd is the problem?

I have a white screen at boot on my HP Pavilion dv4 laptop. When attached to an external vga monitor it works but I'm not sure if its the lcd screen that needs repair. Is there any way to differentiate between video card problems and lcd defects?|||You actually got it right already. Because it worked on external your videocard is good. Had it not been working you wouldn't have gotten video on the external and most likely yor laptop itself would shutoff because of this or be scrambled. So it'll be either your lcd or the wires / circuitry in between that connect it to your laptop motherboard.|||If an external monitor works then it's definitely your LCD panel.