How to find architecture numbers for a GPU model?

Accelerated Computing CUDA CUDA Programming and Performance
1

I have a GeForce GTX 950m, what sm_xx should I use while compiling .cu with nvcc?

2

google it, run deviceQuery CUDA sample code, or check the CUDA article on wikipedia

The xx is just the compute capability expressed as 2 digits. So if you found the compute capability was cc 5.0, you would use sm_50