How can I tell if my new LCD panel is good?

Hi everyone

I bought a used 20" Intel iMac that supposedly only needed a hard drive and a new LCD panel in order to work. The hard drive was easily installed. I ordered a replacement panel off eBay and installed it, but I'm seeing a grey screen with lots of tiny black rectangles on it. I figured that maybe there's a problem with the video card, so I scrounged up a Mini-DVI to DVI adapter, and connected an external monitor. However, nothing ever displays on that external monitor. If there is truly a problem with the video card (the logic board?), I'm going to stop buying parts and sell the carcass....

Anyway, how can I tell if the replacement LCD is good? Does the weird pattern I'm seeing on the display indicate a problem with the panel or with the video card? And shouldnt the iMac automatically output to the external display when I connect it via the adapter?

Thanks!