The best way of finding out if a food is dry is to touch it. It will feel sticky, moist, leathery, or hard. When touching foods for dryness, remember that they feel softer when they are warm. Therefore, always let the foods cool for a few moments -either turn off the dehydrator or remove the drying tray. If you are not sure if an item is sufficiently dry, it is better to overdry it somewhat than to underdry it. However, know that foods that are overdried in some dehydrators may turn brown and become brittle.
If you are concerned about the safety of a dried food, you can freeze it. The freezer will keep frozen any water remaining in the food, thus preventing spoilage. You can freeze dried foods at any stage of the drying process. A woman I once met at a home show told me that she only half-dries her mushrooms because she likes how quickly they rehydrate.