How do I measure of much of something is melted?

Question

For example, I'm making a solar cooker, and I need to record the portion of the food that has melted.

Answer 1

If it has melted it is now a liquid and not a solid.

so if you weigh the food before and after, the difference in weight will be the amount that has melted.

(note: this will not work if you are cooking rice/pasta in water as the rice/pasta absorbs the water so will be heavier afterwards. But it will work for something like butter).