how to tell if the power steering fluid needs to be changed??

how can you tell if it needs to be flushed out? i read that if it is darker in color then it needs to be changed.. what about if you start noticing a noise from the steering wheel when you turn it right or left?

i have a noise comin from my steering wheel it soulds like a rusted spring or maybe squeaking type noise... not too loud but its clearly evident...

what could be the cause?