Partially fill a volume measuring device (for example, a glass measuring cup) with a liquid the candy is not soluble in (for example, oil). Add the candy and note the difference in total volume.
Alternatively, add the candy to a container, and fill the remainder of volume in the container with something that will not dissolve the candy, for example, oil. Remove the candy, and measure the volume of "oil" required to return the container to a "full" condition. (This may give a larger error, due to the "oil" that is removed when the candy is removed.)
With either method, you can use several candies and divide the result by the number of candies to get an average value, if that is of interest.
Alternatively, add the candy to a container, and fill the remainder of volume in the container with something that will not dissolve the candy, for example, oil. Remove the candy, and measure the volume of "oil" required to return the container to a "full" condition. (This may give a larger error, due to the "oil" that is removed when the candy is removed.)
With either method, you can use several candies and divide the result by the number of candies to get an average value, if that is of interest.