Auditory processing disorder is difficult to diagnose because so many of the symptoms are the same for other disorders. I've been told that it takes until a child is about 8 yrs. old to be able to properly diagnose them. I know that younger children have been diagnosed, so I don't believe this is entirely accurate.
The first step that we took was a speech evaluation. That showed a significant developmental language delay so we began speech therapy. At the first appointment, they asked if we had any specific difficulties and after I listed them, the therapist said it sounded like APD, which I'd suspected all along, and tested her for it.
The test of auditory processing disorder was done with headphones. Part was that noise was in the background at a certain decibel level in a certain tone range and then a word was said. She had to repeat the word, thereby differentiating it from the background noise. Another part of the test was that she heard a different word in each ear and had to say which word she heard in her right ear or her left ear only. She had to follow one, two and three step directions, for example, "Say your name, clap your hands, and stand up." Another part was being able to repeat a certain number of words in a row that she heard and/or a sentence. There was more, too, but I can't remember it all.
There are many websites that list symptoms of APD, here's a list taken from one site:
Does your child frequently demonstrate any of the following problems with expressive language?
•Doesn't speak fluently or articulate clearly
•Has poor vocabulary, sentence structure and grammar usage
•Displays illogical flow of stories or ideas
•Uses vague words such as 'thing', 'stuff', 'whatever'
•Problems with receptive language?
•Needs to hear instructions/directions more than once
•Appears overwhelmed when there is a lot of auditory activity
•Misinterprets verbal messages
•Confuses similar words or sounds
•Seems distracted or unable to sustain attention when receiving verbal messages
Problems with other language tasks?
•Cannot associate sounds with their written symbols
•Tends to spell words phonetically (eg. spelling 'fire' as 'fier')
•Reads slowly and has poor reading comprehension
Problems with auditory sensitivity?
•Finds neutral sounds unpleasant or painful
•Puts volume of music or television unusually high or unusually low
Demonstrate any of the following physical coordination problems?
•Poor fine motor skills (using scissors, writing neatly, holding a pencil, etc)
•Poor gross motor skills (catching a ball, skipping, swimming, etc)
•Inability to perform many simple physical activities that others of the same age are able to do
•Falls over and loses balance easily or handles objects clumsily
Demonstrate any of these additional problems?
•Has poor personal organisation (operating within time limits, approaching tasks in a logical order, etc)
•Becomes frustrated, overwhelmed or irritated more easily than most children
•Experiences difficulty with concepts that involve time, direction or sequence