You have probably noticed that some days your moisturizer just vanishes into your skin as if it is thirsty whereas some days it glides on smoothly. When you find that it disappears instantly, this probably means that your you and your skin are dehydrated.
Your skin will absorb moisture where it needs it most so it is easy to see which areas of your skin are the driest by noticing where your moisturizer absorbs the fastest. Of course you can always add a bit more to moisturiser dry areas but the best thing to do is fetch a glass of water!