Bent Derailleur Hanger

Park Tools sells a derailleur hanger tool for just that application. However, depending on the material that the hanger is made of, it may not be possible to bend it (sometimes the case with titanium). Steel and aluminium should be no problem. (unless you have already bent the hanger back many times on an aluminium one - it could suddenly fail) If you have problems with shifting that no amount of fiddling will take care of, it may very well be bent. Any good LBS should have the tool; simply threading it in the hanger and running it around the perimeter of your rim will tell you if it is bent or not.

Quoted post said:

Originally posted by chris_gr
I had my bike inspected at an event and the guy said the derailleur hanger is bent. How do you know if it is? Unfortunately it is not replacable on my 2002 Wilier. Can it be fixed?

Take a look at the Park Tool site at:
http://www.parktool.com/repair_help/DAG.shtml
My experience is that you should "undershoot" adjustment attempts so that you sneak up on the proper alignment. If you overshoot and need to bring it back you will be weakening the hanger more thannecessary.
This is a job for experienced hands and/or someone who understands the principles involved.