I just got a new laptop that has Nvidia Optimus. It's got an Intel GTX 960m and it also has Intel HD Graphics 530. I installed the latest Kali version inside VirtualBox. The laptop itself is running Windows 10, if that makes a difference. When I run the lspci command in terminal, this is the only graphics controller that's listed:

00:02.0 VGA compatible controller: InnoTek Systemberatung GmbH VirtualBox Graphics Adapter (prog-if 00 [VGA controller])
Flags: bus master, fast devsel, latency 0, IRQ 18
Memory at e0000000 (32-bit, prefetchable) [size=128M]
[virtual] Expansion ROM at 000c0000 [disabled] [size=128K]
Kernel driver in use: vboxvideo
Kernel modules: vboxvideo

I tried following some guides, but they all seem to depend on my GTX 960m or Intel Graphics actually showing up in this list, but they aren't. I can't tell if it's using the 960m or Intel graphics, I don't know if I can switch between them etc. I know there are some tools that utilize CUDA cores, which would be cool if I could use the 960m for that if I ever want to mess with them, but in general I'd like it to use the Intel HD Graphics.

What is the best way to make sure all of this is configured correctly?

I'm on the latest version, I just ran updates, upgrades, dist-upgrades, updated the headers, and installed the guest additions. So everything is as up to date as possible.