Do you want to test the glaze or whether lead leaches from the glaze?
To test the glaze itself, you'll have to dissolve it, even if you're using a home test kit. Those kits are usually intended for use with, e.g., drinking water. In a professional lab you'd use an acid digestion, which would be difficult to do safely at home. Then you'd use an ICP-MS or similar equipment. You could use some other analytic tests or kits but they're not as sensitive and may not work at all in highly acidified solutions.