Coolingconfigurator.com like rubix said. It's a site by EK (A waterblock and liquid cooling component manufacturer) that allows you to put in your GPU, CPU, or MB brand and model and will tell you whether or not it is
A) A reference design model
B) A non-reference design model which EK has made a custom block for
If your GPU falls under category A (which you can tell because at the top it will have a photo of your card less the heatsink and will say reference design) then any waterblock made for that specific series (680, 670 etc.) will be compatible with your card.
Hope that sheds some more light.