Since there are no commercial transactions on Couchsurfing, the site uses other ways to verify that people are who they say they are. Paying $25 will match the name you put on the site with the name you use with your bank. If you do that, you'll get the words "identity checked" next to your profile. Couchsurfing then mails a postcard with a special code to the address you provided. Once you enter the code on the website, you are "verified." We were more likely to accept verified guests, though it wasn't a requirement.
I also connected my Facebook account with my Airbnb and Couchsurfing profiles and checked out the Facebook pages of potential hosts and guests. We even used some interest-based filters to vet potential Couchsurfing guests. For example, having been to the annual Burning Man event in Nevada will likely get you in our door, while other hosts might automatically disqualify you for that.
The more you use the sites, the more friends and reviews you get from other users, which further serves to ensure trust. It's a bit like eBay, and how your cred improves the more transactions you do and do well.
The rest comes down to gut feelings. We made it clear to potential guests that we live in a small apartment and that they'd be sleeping on our fold-out futon in a living room with no doors, possibly with one or two cats on top of them. On weekends, we stroll around the city to explore new and old sights, scenes and tastes.
If that's not your cup of tea, the Hilton in Times Square might be a better bet. I won't forgo hotels now that I've tried out Couchsurfing and Airbnb, but it's good to know the options are out there. It certainly makes travel — and staying home— more interesting.