How to tell if an empty PCI Express slot is 1.0, 2.0 etc? (general test, or enquiry command, not refer to spec)

Use "System Information for Windows":

http://www.gtopala.com/

One of the best tools I have ever used.

Accepting @Breakthrough's answer for HWiNFO32. Screenshot is actually conclusive (correcting my initial findings) - I have PCI Express 1.0 - it states that explicitly. This makes sense to me given that the motherboard is about 3 years+ old. Screenshot of HWiNFO32 showing the version of the PCI slot:

@Breakthrough - your comment above would be the accepted answer. I've created this on your behalf but if you write your comment as an answer I will move the 'tick'/'check' accepted answer to yours and also +1 it :)

I would also suggest GPU-Z. The name and interface are very similar to CPU-Z, but it's optimized specifically for video card info. PCI Express version is shown in the "Bus Interface" box in the main tab.