There are slender white tail markings, very different than the bold white tail spots of a Yellow-billed Cuckoo.
Birders went cuckoo today over the sighting of a migrant Black-billed Cuckoo at the Sanibel Lighthouse park, Sanibel, FL. This is a rare bird for here and many birders and tourists lined up to get a view or photograph while keeping a respectful distance from the bird. The cuckoo put on a show and posed in the shrubs, a few times going down into the grass to grab a small lizard for a snack.
We see Black-billed Cuckoos on our NH property although not very frequently. Here in Sanibel, where they have Mangrove Cuckoos in Ding Darling NWR, and Yellow-billed Cuckoos as migrants, the Black-billed Cuckoo is the one that steals the show.