Boy thats a tuff one! Personal descriptions written on boxes can be troublesome. When the box says "clothes" or "yard sale", I believe what it says. When the box says "Grannies collectibles", well that could mean anything. Are we talking fine antiques or Dollar Tree figurines? I usually try to compare what loose items are laying around the unit and perhaps sticking out of the box to make an educated guess as to what is in those boxes.
Now original merchandise boxes are a different story. When I see LG flatscreen LCD TV, or Playstation 3, or Dyson vacuum, etc. I never assume that the original product is still in those boxes. I assume they are filled with garbage, dirty clothes, or are just empty, unless they are close enough to the door to get a better look.
Only one time I took a chance on an original merchandise box. It was at a piece auction. They were pulling out boxes of personal items out of moving crates and making lots of 10 boxes or so. That day, the lots were being bid as high as the whole moving crate would have went for at a normal auction. One lot got put together and one of the boxes I recognized as labeled with the name of a medical device manufacturer. We were allowed to walk around the boxes/lots but not look inside the boxes. The box I was interested in looked factory sealed so I took a chance and won that lot. The box paid off, it was a piece of medical equipment that paid for the lot and then some, but if it hadn't been there I would have lost money on everything else left in the lot of boxes.