How deep should we be in the skin?  It does depend on the area your tattooing.  As an example the eyebrows verses the eyelids are different.  Even the eyelids at the tarsal plate are different than the eyelid skin above it.

As a former skin care professional, I can just about look at the skin and know if it's thick or thin, however you can also give it a pinch and feel the skin and note the difference.  Tattooing in thin skin will be more shallow than tattooing in thicker skin.  The best advice I can give is to feel your needles in the skin with your stretching fingers.  It's something you start to intuitively do and you know where you are in the skin by feeling the vibration form the needles.  

It's more difficult to feel smaller needle groupings and also smaller diameter needles such as nano needles.  The vibration is minimal and the nano needles  and smaller groups penetrate the skin quickly so it's important to F E E L where your...