I can't tell if I have a regular pcie 16x or if it is four wired

[Soccer king]

I currently have a dell xps 8900 desktop with 8 gb ram, i7 6700, 1tb, with a 960 ftw gpu... The problem I'm facing is determining if the other pcie 16x slot is "4 wired" or not. I am looking to add another gpu, same as above, in a sli format. However, I want to make sure that my mother board is capable of having another before investing. I'm really sorry for the trouble, this is my first gaming desktop, I'm new to this... Any and all help is appreciated. Thanks, and my apologies if applicable

 

[bignastyid]

http://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_xps_desktop/xps-8900-desktop_reference%20guide_en-us.pdf

2nd 16x slot only runs at 4x, the motherboard doesn't support SLI.