Sneak

The character becomes invisible to all monsters that fail a Spot and Listen skill check versus your Hide and Move Silently skills. These skills do nothing unless you are in Sneak mode.

Additionally the number of eyes next to your character indicate how visible you are (1 to 5 eyes, 5 being the most visible, you may need to zoom out to see these).

Also when moving across certain terrain or rubble red splashes will appear at your feet, indicating you are making noise, again you may need to zoom out to see these.