You know it's time to move on from your present job when you see these things happening:
Lack of interest
The challenge has gone out of the job and things are becoming routine and mundane.
Lack of trust
You notice that you are not getting the same respect as before and things you do are being questioned more often.
Lack of fun
When the fun is gone then it becomes work and that is no fun at all.
Over the years you start to see what things about your job spark the most interest. For instance I really like organizing things and have a little jump to my step whenever I start the planning process for an event. I also enjoy training and passing on information. I am in the process of developing testing for admin interviews and I really enjoyed doing that. I also like creating job manuals. To me it is fun to put the pieces together and come up with a manual that will be helpful to any new person starting the job (I think that goes along with my enjoyment of training). I enjoy taking minutes and all that goes with it (I guess that is why I love giving the minute-taking webinars). But there are also aspects of my job that I don't like as much and that will be normal for any job. If you have a 60/40 split you are doing well. I would say in my current job it is probably an 80/20 so that is pretty good.
It is always good to do an evaluation of your job and you may come away from the exercise deciding to put your resume out and look for something else or you may come away encouraged in your present job.
I have worked with a few people who applied for jobs and then when they were offered it, passed on the opportunity. I think they didn't do their homework about whether they really wanted to move on or not. If you honestly evaluate your current position and find it wanting, then you can go ahead and start looking with a clear objective of securing another job, but if you start applying before you do that you may just end up going through the exercise and not accomplishing anything (although there is something to be said about going on interviews just for the practice and keeping up to date on current interview techniques etc.).
Other things you need to evaluate as well is the salary, career advancement, company benefits, etc., but an evaluation on the enjoyment part of your job is a good basis for either staying or going.