If you can't get the information directly from the airlines website, sometime I use
www.orbitz.com or
www.americanexpress.com (go to the travel link on their site). You have to go through the process of selecting your flight and putting in the passengers names, as if you want to make a purchase, but before they ask for any credit card info you are taken to the seat selection screen - there you should be able to see how full the flight is without having to complete the purchase.
HTH!