How can I tell if my idler arm needs replaced?

Is your Idler shaking or has play or does belt wear quickly or cut..you'll know when car runs, you'll see a wobble

It's a Jimmy; it needs a new one. But, to be sure: is there a lot of play in the steering wheel when you're sitting still? You can also jack the front of the car up and trying rocking the tire side to side laterally. If there's a lot of movement, it needs to be replaced. Take a look at your pitman arm as well while you're down there; I find they usually go together.

jack up the front end and try to shake it. If it has play in it then it's bad. Chances are if the idler arm is bad and nothing else has been replaced then other parts of the steering linkage may be going as well. Rag joint, pitman arm, tie rods, and so on.