The standard does not specify if atomic objects are lock-free. On a platform that doesn’t provide lock-free atomic operations for a type T, atomic<T> objects may be implemented using a mutex, which wouldn’t be lock-free. In that case, any containers using these objects in their implementation would not be lock-free either.
The standard does provide a way to check if an atomic<T> variable is lock-free: you can use var.is_lock_free() or atomic_is_lock_free(&var). These functions are guaranteed to always return the same value for the same type T on a given program execution. For basic types such as int, There are also macros provided (e.g. ATOMIC_INT_LOCK_FREE) which specify if lock-free atomic access to that type is available.