What does sugar water honey look like?
If you go to the other site Lauri has some excellent photos comparing capped honey to capped sugar water. The problem is when it gets mixed you can't tell visually anymore. You can get away with selling honey that is contaminated with sugar water at a farmers stand but if anyone gets wind of it you won't sell much in the future no matter how pure it is.
Commercial beekeepers know what they are doing so if there is some mixing it is infinitesimal or it is on purpose. So as a back yard beek if your bees don't need feed don't give it to them.
I have read a lot of posts from newbies and I can say with a lot of conviction that worrying solves nothing and most often causes a newbie to do the wrong thing.
I don't know if you watch NCIS and are tuned into Gibs rules but if he was a beekeeper rule #1 would be: If you don't know, you don't do.