How do I check what GPU is active on Windows?

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C++

Windows

OpenGL

CUDA

GPU

I need a quick brush up on how I can conditionally (pre-process) code and check to see what GPU is installed or the such. Why I need this is because I am writing a program that is expected to be very cross-platform and capable of having workarounds on mostly all modern hardware. For GPGPU specifically, I can use CUDA if NVidia is detected; however, it can't be used on non-NVidia GPUs, so I'd need to conditionally check this (and add OpenCL or some other library to aid in GPGPU).

I will learn with other OSes, but for now, how can I do this on Windows? I've searched NVidia's website and they say nothing about determining CUDA in pre-compilation (without having it installed, which it needs not be). Here's a pseudo-way of looking at it:

IF (ANY) NVIDIA GPU AND OR OPENGL 4 - > INCLUDE CUDA LIBRARY AND IMPLEMENT IT.

IF (ANY) NVIDIA GPU NOT DETECTED - > INCLUDE OPENCL AND IMPLEMENT IT.

IF AMD ATI RADEON 7x/8x VERSION - > INCLUDE MANTLE API AND OPENCL.

I need to set up conditional compiling schemes so that others can safely test and execute the program as it grows on different platforms, GPUs, etc.