Indeed gpu-z is the way to go, but have something rendering in a window or generally have some load on the card, otherwise it will be showing pcie1.1 or pci2.0 or some other low load reading.
Yup. There's actually a little doodad in GPU-z that will do that for you. I think it's a question mark you click next to interface. Guess there were a lot of questions about why PCI-Express 2/3 was showing up as 1.1.