This is a really good question and one that you will often hear from customers wanting to buy your functional pottery. Fortunately, there are a lot of tried and true glazes available out there but that doesn't necessarily mean that they will be safe with your clay. In order to have absolute certainty I recommend having your clay and glazes professionally tested which can be a costly affair but if you are selling functional dinnerware to the public, it's a safe investment.
Concerning our native clay, I have covered the shrinkage rates as well as the rate of absorption in my previous postings. Now we want to test for the absorption rate of your clay and glaze. In order to do this you need to completely glaze your test tiles and fire to maturing temperature in the kiln. In the realm of earthenware clay your bisque firing temperature is normally 1 to 2 cones hotter than the glaze firing temperature.
After completion of the glaze firing we will again measure our 100mm lines and weigh the tiles. The final lengths of the tiles will determine our overall shrinkage rate from dry to glaze for our native clay. Again, under 15% is good. Next you will need to boil the glazed test tiles in water for 1 hour. Safely remove the tiles from the hot water and allow them to cool to the touch before weighing them again. The difference and average of dry glazed test tiles to the saturated glazed tiles will allow us to determine the rate of absorption of our clay and glaze. If your percentage averages out below 3% you have a good clay and glaze fit. Chances are very good that this clay and glaze is food and water safe. Higher percentages will result in weeping water as well as wares that are unsanitary.
A final test involves acidity. Take one test tile and soak it in vinegar or lemon juice for 24 hours. If the glaze is discolored and showing craze lines your glaze is not safe for acidic foods.
If you happen to have a poor clay and glaze fit you can still create wares that are not crafted for food and drink as well as test with other glazes.