This is evident by a change the gloss and texture of the glass surface over time. The ability of the glaze to pass two simple tests can be a good assurance that it will give trouble-free service. Compare the surfaces of the two very closely. If the washed one looks any different, either in color, gloss, or texture then you have a soluble glaze. Dry the item and compare the color and gloss of the surface above and below the liquid line.
Any difference indicates that the glaze is subject to leaching.
Excessively unstable glazes can easily turn white with an overnight vinegar leach. If you want to be a little more scientific you can send the vinegar leachate to a lab to test for the metal in question, then compare the result with its drinking water standards. This type of information is available by using a search engine on the Internet. Remember also that to be totally applicable someone would have to drink all their water from the same glazed container, and any metals already in the water, body weight, whether or not the body accumulates it, etc. would have to be taken into account. These tests are not technical enough to guarantee that ware is going to be absolutely safe but they will certainly expose glazes that are obviously unsafe.