I tend to look at the price on ebay. if someone is selling chanel no. 5 parfum for 10 dollars you know it is going to be a fake; whereas if the retail price is say 100 and they're selling for like 70 odd that would be okay imho; also check the seller's rating.
i ordered a v small (7.5 ml/0.25 oz) bottle of joy parfum from ebay a few days ago for about 80 euros; it hasnt arrived yet but the bottle and the seller looked reputable and so did the price. i tend to find things from europe are usually genuine. i guess it is a gamble at the end of the day...but you can minimize risk with a bit of common sense.