How do you identify a gold watch and the quantity of gold? Does it always have something like "18K", "14K", etc. inscribed somewhere?

If it is gold-filled or gold-plated, how do you differentiate that?

 

Sometimes there are markings on the back edge of the case or between the lugs, but most times (particularly for Omega) it's on the inside of the case back. For instance "20 m plaque" means 20 microns of gold plating.