There are many factors involved in stealth calculation :
Sneak Skill, obviously. This can be improved temporarily or permanently by perks and items.
Enemy Perception stat : it is worth noting that a crippled head cause a -4 modifier
Light level : less light = more difficult to see. Be careful for your Pip-Boy light.
Movement : this includes looking around. The more you move, the more you are likely to be detected.
Contact : usurprisingly, being in direct physical contact with any target makes it aware of your position
Sound : you emit sound in two ways :
- Most weapons are noisy. Some are silenced : they don't emit sound when you use them ; that includes melee weapons and grenades (throwing is silent). Being attacked will always cause a NPC to enter [CAUTION] mode.
- Moving emits sound, which scales with your equipped weight ; this only includes your weapon if it is drawn. The perk "Silent Runner" negates this entirely.
Line of sight : if you cannot be seen, you can only be heard.