It's a good idea to have them complete some type of questionnaire when you give them soap to test and ask them to answer the questions truthfully. If they didn't like something about the soap, make sure they tell you what it was. Otherwise all you'll get is "I liked it" which doesn't really tell you anything. Make sure they know that they won't get any other test soaps if they don't complete the questionnaire. You can ask questions like: was the soap too drying, did it moisturize, any itching, rashes, etc after using, was the scent to strong, not strong enough, how long did the bar last, was the lather bubbly or creamy, not enough lather...you get the idea. Pretty much anything that will help you develop a better bar of soap. And don't be discouraged if some of the responses aren't positive. That is why you are doing the testing.