This is a great question, there are some very evident signs when skin is dry. Flakiness - when you pet or brush your dog you will see flakes that look like dandruff. Along with flaking, the coat tends to be dry. Visually, the coat may look dull not shiny. Groomers know what they're looking at, you can also ask your veterinarian. By asking your vet, you can rule out a medical condition. Overbathing can cause dry skin too. If the dry skin is not related to a medical condition, using a soothing type of shampoo one with aloe or oatmeal, plus a cooridinating conditioner can help. When you bathe, rinse well beyond the time you think the shampoo is gone. Excess shampoo or not rinsing completely can irritate the skin causing it to become dry. Adding Omega 3/fish oil to your dog's diet helps with dry skin and coat.