1) You have no motivation. To do your job well, you must be motivated. It is truly as simple as that. If you find yourself lacking motivation to do the simplest parts of your job, it is a definitely sign that you are working at the wrong place.
2) You hate what you do. Try this: Make a list of things you like about your job and another of things you absolutely hate about what you do. If the cons are definitely more than the pros, you should probably look for a new job!
3) You don't feel challenged at work. Even if your job is not exciting, you should still feel challenged during tasks. If you find yourself feeling 'bored' or feeling like your skills are not being utilized the way they should be, get out asap!
4) You constantly complain about your job. Complaining about your job here and there is totally normal. However, if your family and friends are tired of hearing you constantly bash your job, consider looking for something new.
5) You see no signs of growth. If you have been doing the same job for years with no promotion opportunity in sight, it may be a good idea to find a company that has growth potential.