how can you tell if a idler is worn?

 

excessive movement, try to move it with your hand or have someone turn the stearing wheel and watch for excessive movement

 

Yea, what Ede said. What I could do with mine was after putting it up on jack stands, slide under on my back and grab where the center link and tie rod join on the passenger side. I could move it up & down a half inch or so. The answers from my Q's on the board said that was pretty bad. It took me about 2 hrs to replace and didn't lose any blood.
I was also advised to be sure the when installing the new one to adjust it to get the centerlink level. I used a bubble level and a ruler, worked for me. Just do it !