Considering an NPC is done by right-clicking on it or targeting it and pressing the "c" key on your keyboard.
Consider yields two kinds of information. The color of the consider message is used to determine the relative level of the NPC. The message itself reflects their attitude. For more information on NPC's attitude towards you, see article the on factions.
|Below Your Level||At Your Level||Above Your Level|
|Grey||Green||Light Blue||Dark Blue||White||Yellow||Red|
Grey NPCs will give no experience, Green kills will give very little experience, while Light Blue kills will give less than full (but still decent) experience solo, but very little to a group.
Dark Blue, White, Yellow, Red give full experience to both solo and grouped players. However, once a player character gains a certain amount of Alternate Advancements, enemies of lower difficulty will begin giving less experience. This penalty increases with the number of Alternate Advancements purchased.