How to test oxalic acid in potatoes
If you want to determine the total content of oxalate you would need to extract it from the matrix with hot HCl 1M, for instance, since it will dissolve calcium oxalate crystals. Afterwords you can precipitate oxalate again at pH=4-5 with calcium nitrate, thus forming white crystals of calcium oxalate. You can redissolve them by adding sulfuric acid and identify oxalate by taking advantage of its reducing properties - if you add potassium permanganate it will be reduced from Mn +7 (purple colored) to Mn +2 (brown colour) and oxalate will be oxidized to carbon dioxide. But of course you start with a complex matrix, the easiest thing would be to use chromatography to separate and identify oxalate.