So I have 2 NVIDIA GeForce GT 650M inside my notebook. I got SLI enabled in the NVIDIA Control Panel, however I suspect, from the external heat of my machine, that only one of them is used. I downloaded HWMonitor, to be sure of this. It seems that I was right, is it broken or I just missed something out of my sight?
In Nvidia's drivers there is an SLI visual indicator (or at least there used to be).
Also, another option is to try some of the GPU vendor tools, like precisionx.