The Sneak skill determines how well one can use sneak mode to sneak around NPCs. If one manages to sneak upon a character and then attack them, there is a chance of a critical hit. To tell whether or not the Nerevarine is visible/undetected while sneaking, there is an icon near the bottom. This icon displays the skill symbol for Sneak, a hand reaching for a bag. If the icon is not present, they have detected the Nerevarine.