Wearing a ring of teleport control (or ring of TC) conveys the teleport control extrinsic. This is an incredibly useful property, especially when combined with teleportitis. Eating a ring of teleport control grants the property as an intrinsic.
One of the ways to identify a ring of teleport control is to teleport (usually with a teleport trap or scroll) while wearing an (ideally noncursed) ring. If you can choose your destination, you have the teleport control ability. Dropping a ring of teleport control into a sink produces the message, "The sink looks like it is being beamed aboard somewhere."