They can ask your web browser what GPU you have.

If it says QEMU, the jig is up.

See here:

https://developer.mozilla.org/en-US/docs/Web/API/GPUAdapterI...

Live demo here:

https://www.checkadevice.com/tests/gpu/ (random web site that came up in a quick search)

You have to make sure your VM + GPU hide or falsify all identity information. They just have to find one API or signal that you missed that reveals it.