If I'm using somebody elses' cards, I might judge by the box.
But as for my cards, I use a card clip so my boxes are usually in good condition, which would make it difficult to judge the cards by.
I'd look at the edges first. A new deck will have nice white edges, if it's a black deck it'll have nice black edges. If it's worn, the white will have dirty brown edges and the black will bad looking white edges.
I'd then do a circle fan in the right hand (no pips) to see more clearly how much dirt is on the edges.
But before any of this happens, the #1 thing I judge cards on is how the edges feel... followed by how well they fan
Hope I helped
ZG