Usually its Intel + ATI CrossfireX or AMD + NVidia SLI. Though there are some boards that do both (I stick with Intel/ATI, so not too sure on which do both sorry )
Its always been Nvidia + Nvidia SLi and Intel and AMD + ATi! Before Core i7 SLi was only officially possible on Nvidia chipsets, be it for Intel or AMD processors. Now some X58 boards support crossfire and SLi as Nvidia has effectively been forced to licence the tech by being squeezed out of the chipset market.
Only way to tell is look at the motherboard features list - it will say.