Install nvidia-smi on OSX
In Macs there is no nvidia-smi command that comes with nvidia drivers. However, you could check this open-source alternative : https://github.com/phvu/cuda-smi

Edit in 2020 : 
As an alternative to nvidia-smi is Activity Monitor. Search on spotlight (cmd + space) and type Activity Monitor. When the program opens press cmd + 4. This will show you the active usage of GPU(s) on your system.
I think this feature comes with High Sierra 10.13 prior to that there is no option for gpu history.