If you want to find out what they are worth, you dont have to find out what motherboard it is, just what it supports.

First look at the cpu socket. Intel or AMD? slot or socket?
Check the ram. If there are 3 sections to the ram slots, its sdram, 2 its DDR.

Are the ram slots evenly spread, or is there one farther out from the others. If there is, its probably dual channel.

Look at the northbridge. If you take off the heatsink (provided there is one), the chipset will probably be listed. Sometimes, the chipset type is even printed on the board.

you can get a pretty good idea of what the motherboard is worth just by checking out these things.