Is there a gadget I can buy that I can plug into a USB port (a data port, not just some DC source) and find out if it is USB 2, 3.0, 3.1 or 4 etc? There are gadgets that can tell me some details, but not the actual standard supported.

 

This tester might do what you want. It is startlingly comprehensive, and the manual is not native English, but this screenshot:

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shows that it does include what they are calling the "charging agreement" (USB 2.0 FULL, in this case) which is negotiated and that seems to be just what you want. I've attached the manual so you can see the amazing array of functions this thing has.

I went ahead and ordered one because I needed a USB C capable tester, I will let you know how it works out but it certainly looks good.