I have zero experience of buying a used Rohloff, but it occurs to me that I'd rather have seen it in the wheel right after it was taken out of the bike, or at least filthy dirty right after it was cut out, because then leaks would be obvious. Once it's been cleaned up, there's not much you can tell except, possibly, that there are 14 gears.
If it is a pre-laser-serial model, the blue and yellow sticker should be attached. If it is not, don't buy it because you may not be able to get service.
You should know that the normal ownership for a Rohloff isn't genteel tourers but mud plugging offroaders, so a secondhand Rohloff, unless you're able reliably to determine its previous ownership and use, or unless it is visually pristine, is very likely to have led a hard, dirty life. That's why people try to buy a pre-loved Rohloff within the forum or club or from a source where the prior use can be guessed at with some accuracy.