Many confuse acid marks with sandblasting which has virtually replaced acid marking as it is much cheaper. This will, in turn be giving way to inkjet printed markings which are currently used in the container industry and bound to cross over to other glassware soon if not already.
Acid marks are smoothly textured and can be continuous designs using a stamp but a stencil could be used. Sandblasted is always stencilled, a quick but not certain give away, but under a glass you will see coarse indents in the surface of the glass. Too early to give guidance on inkjet marks as the technology is developing fast now - but could be etchant, enamels or more likely chemically-bonding epoxy inks that could potentially get confused with acid marks.
Stuart were using sandblasting for signatures for quite a while, often they are very lightly marked and sometimes completely missing. I get the impression they rushed the marking.
Edinburgh Crystal sandblast marks by comparison are invariably more heavily applied. But they certainly left some pieces unmarked, I am told, as a quality issue. But, some of imported EC glass with marks does exhibit poor quality.