The Tell is a physical aspect of a Lunar Exalt's Spirit Shape that manifests in his human form and, to a far more subtle and hard-to-notice degree, in other forms. Some Lunars believe the Tell is an integral aspect of their being, holding their protean forms together, and the Silver Pact is fond of including them in moonsilver tattoo designs. A Tell can take any form so long as mortal senses can perceive it.
Similar to the Spirit Shape, the Tell relates to a Lunar's personality and reflects an appropriate animal quality. For example, a perceptive Lunar with a chameleon Totem may have eyes that move independently, an unmannered boar might smell of her totem animal, and a socialite Lunar's voice echoes with birdsong. Visual tells are often silver or white, or reflect moonlight.
Tells manifest no matter what shape the Lunar takes, however they can. They at least always follow a theme - A Tell of hooves may not show on other forms - but their footsteps click on the ground like hooves normally would. Feathers across one's back may become a feather-like pattern on fur, or even markings on scales, if not actual feathers. A scorpion barb may be hidden under armored plates, or become a strange hook on the end of a cat tail.
Despite all of this, Tells are not easy to detect. A Lunar would never survive if they were that easily recognized. Instead, Lunars radiate an unnoticed mental command influencing people to ignore the otherwise obvious Tell. Tells become more obvious as a Lunar expends motes, manifesting along with her caste mark and anima banner.
A Lunar’s Tell is considered an Illusion effect. Observers explicitly cannot spend Willpower to see through a Lunar’s Tell; only Illusion-negating magic and sufficiently high MDVs are capable of piercing the layers of misdirection Luna granted to her Chosen.
Source: Scroll of Errata