One quick way (but not too reliable) to tell if the LNB arm is bent is to push it up and down slightly by hand as you watch the signal quality. If it improves as you bend it then you know the dish is bent. But to be honest, when I run into a situation like that I just slap up an new dish - I mean they are cheap in comparison to what a customer would pay me for playing around trying to fix a bent dish...
kat