I'd wager it being 16, since it's a more common size to work from. Most common metal tools are only rated to 16 gauge mild, or 14 gauge stainless (smaller gauge number = thicker). If it's 14 gauge, if the shield is roughly 24" x 36", the 14 gauge will only be ~10 to 15% heavier than 16 gauge. If it's a non-combat shield, I'd actually bet on it being 18 gauge - that's what I usually see in the "catalog" shopping. Either way, it would be a nominal difference in weight, unless you're fighting with it for hours on end. Strapped over the shoulder or back? I doubt you'd notice any difference at all.
Could always plop it on a bathroom scale for the weight.