Apple maggots are the apple tree growers enemy. They can easily destroy an entire crop. Obvious damage is seen as brown spots or trails within an apple. You may also come across the maggot itself. If you suspect you may have this infestation but aren't sure, pick an apple, slice in half and put in a glass jar for a day or so. The lack of oxygen will force the maggot out if it is still present.
Kudos to my friend Kim who recently posted one of the best articles on apple maggot identification I have read to date. Check out her site here: http://meadowoftheunicorn.blogspot.ca/2012/09/the-apple-umbrella-tree.html. It is complete with photos and instructions on what not to do and what to do if you have this pest.
There are apple maggot traps available as well. Those that I am familiar with resemble apples in colour (red) and are spherical. To this ball you apply a sticky substance (included with the kit) and then hang this on the affected tree. The red attracts the fly which will then become stuck in the substance. You may want to hang several in your tree/s. Remember to clean up any debris including fallen apples, to prevent the maggot from overwintering in the litter and continuing the cycle the following season.
Get your neighbours involved as well to increase your chances of eliminating this pest and preventing it from reaching the orchards of British Columbia. Spread the word.