The following is a how-to describing how I've learned to tell Zwilling J.A. Henckels Professional S series factory seconds, from NIB retail knives. 

The term "Factory Seconds" (or just "seconds") refers to the quality of the item, a NIB retail-boxed item can be expected to have gone through a degree of quality assurance before being shipped to a store - if that level of quality wasn't met, anywhere along the production line, the manufacturer has 3 major choices;

1. recycle the parts, and return to the beginning.
2. Throw out / Trash the item (expensive and a waste)
   
3. Sell the substandard item, at a reduced cost, and clearly mark that it is of a lower quality than usual.

Henckels identifies knives during their production cycle which do not meet their stringent quality demands, and label them 'second choice' (Their way of stating the item is below-par).  'Second choice' knives are of lower retail value, having found a flaw during the production cycle - sometimes those flaws cannot be seen, like the quality or metallurgical make-up of the steel used in the blade.  While these flaws were detected too far along the production cycle for the components of the knife to be recycled back into the manufacturing process, they are not of poor enough quality to be thrown out - Henckels chooses to sell them at a discounted rate.

Henckels seconds can be distinguished in 4 ways, that I've noticed;
a) The Model # on the plastic heat sealed casing it is boxed in ends with a -5, the model information on the knife itself does not have the -5, only the box
    BEWARE: I have seen "NWOT" items suspiciously listed "without box" potentially in an effort to cover up the model number.
b) The blade has an 'B' stamped on it at the base of the blade on the same side as the model number.
    BEWARE of sellers who do not show you a picture of the side of the knife with the model number
    BEWARE of sellers who only take pictures of part of the blade (like a tight close up of the model info, the B is a half inch right of that writing, just before the handle.)
c) The knives do not ship in the usual Henckel's box, but do ship in a heat-sealed hard plastic case, with a problem black and red Henckels cardboard insert.
    BEWARE Regular retail packaging is a plastic rectangular box, not heat sealed, that can be opened from either end, with molded plastic and cardboard inserts.
d) A ~1.25 inch black and white sticker saying 'SECOND CHOICE - NO WARRANTY' is stuck to the outside of the clear plastic heat sealed case, but is easily peeled off.