Read and follow the iMac Intel 20" EMC 2105 and 2118 Display Replacement guide, and remove your screen completely.

Flip the screen over, placing it on a towel to keep it safe. Locate the manufacturer's sticker on the back of the screen, which should contain the screen's Model Number

The screen used in this particular iMac 20" A#### EMC#### is an LG-brand model-LM201W01, though yours may be different.