About all you can do is look for branded motherboards that physically appear identical to the motherboard you have.
There may be a clue or two in the fine print on the motherboard.
If the mother board came from a Lenovo system that may help narrow the search.
However there may be some modifications to the board for Lenovo's (or whatever manufacturer) purposes.
Take a couple of photographs - ensure that they are clear and sharp.
Post accordingly. Someone may recognize the board.
Here is how to post an image:
http://www.tomshardware.com/answers/id-2173703/post-images-tomshardware-guide.html