No, no such guide exists. At least not in the UK, and I'm not aware of any elsewhere in the world.
As with used organs, you need to check every key for correct operation, with the touch response enabled. Press every button and see if they all work. Play it loud and see if there's any distortion. Plug in headphones to make sure that socket works. Look at the power supply block. Is it, and the cables, in good order. Does the power lead fit well in the socket. Check the screen - certain models were known for having problems. It should be clear, no odd colours, lines or blotches. If you can, and you know how, check that the USB socket is OK, load and save something. If the keyboard's old and has a floppy drive, be wary, replacements may be unobtainable as some are non-standard.
If you can, take someone along who knows what they are looking at. If in doubt, walk away.
Buying from a dealer will cost you more, of course, but you do have recourse if there are any problems.