Haworthiopsis fasciata is rare in cultivation, and can easily be distinguished by the smooth upper (ie. inner) surfaces of its leaves. Its white tubercles occur only on the lower (outer) sides of its leaves; whereas H. attenuata has roughness or tubercles on both sides of its leaves. The leaves of H. fasciata are also often stouter and more deltoid. They tend to curve inwards more. Unlike H. attenuata, older H. fasciata specimens also sometimes develop long columnal stems.
The most fundamental distinction, though not externally obvious, is that Haworthiopsis fasciata has fibrous leaves – unlike H. attenuata.
Source: Plant Rescue