The Detect Life spell allows the user to distinguish between friendly and hostile characters through the color of the mist it uses to show their presence — hostile characters are red, and friendly characters are light blue.
Note that the blue mist surrounding what appears to be a friendly character/creature may become red mist as the Dragonborn approaches and the character/creature becomes hostile on sight.
Useful for stealth-oriented classes, as it can be used to find out how many foes are in the next room, then calculate an efficient plan of attack.
Can be extremely efficient at raising Alteration, as it increases the amount of experience gain the more targets it is able to see. For example, seeing one person may take a couple of minutes to increase, while seeing two people will cut that in half. So to get the most of this spell, cast it in a major city such as the Whiterun or Riften markets.