"Consider" or "Con" is the way for adventurers to get a feel for how difficult a creature will be to kill. Consider works by judging the creatures level versus your level to give you an idea of how you would match up.
Consider ranges from "Like stepping on a bug..." (Easiest) to "You ARE mad!" (Hardest) You generally dont want to attack creatures that you consider at "You ARE mad!" On the other hand, creatures that you consider at "Like stepping on a bug" are typically not going to give you much experience.
Example: I'm at the Elkin fountain on a level 75 character. I see a Street Sweeper. I type: "con sweeper" and it says "Like stepping on a bug..." On the other hand, I go to Groulth (e, n5) And I "con grouth" it says "You would need some luck!" That is self explanatory, I'd have to be pretty lucky to kill Grouth.
Note: This is by no means a sure fire way of knowing what you can and cannot kill. This is strictly a comparison of levels, some creatures are more difficult than others even if they are the same level.