I ask people how they found me. Most of the time I stick with the free listings until I get a few clients who've found me through a particular website/listing, and then I might upgrade to a paid listing. It's my experience that traffic patterns change, and it's important to pay attention to whether people continue to be referred by the pay listings. I've had several sites where my paid listings started out hot, but cooled down after a year of so. If the site is not working hard to keep their traffic and search engine rankings up then people won't find your listing.
I've pretty much decided I'm done with paid listings on supposedly local but really national websites. I am not getting enough business from them to justify the hundreds of $$$ they charge. I'd be interested in paying for a listing on a good local website, if there was one. My local paper's website has an online bridal guide, but I don't think many brides know about it.
At least 2/3rds of my business comes from recommendations from previous clients, and networking with other wedding vendors. I let a local, upscale hotel use photos no charge from weddings I've shot at the hotel in ads and promotional materials, and they put me on their link list, and often recommend me in person to people. Some might gripe at me for "working for free", but in my opinion it's a valuable trade. They are sending me a lot of business.