BoiGeorge
In my opinion animagus may not necessarily have a magical signature, but they would gain an aura that separates them from other magical people when human and animals when transformed. Most would not be able to tell, some on instinct can feel an animalistic vibe from a person or a difference if they see their transformation. A few maybe able to sense the aura that surrounds an animagus and certainly most of the competent animagus would be able to sense another animagus at least in their transformed state if they see them. Then you have Oz and his new eyes that should allow him to see that someone is an animagus in both human and transformed state even if he doesn't know who the person is if seeing the transformed state or the animagus form of a person he sees. (Note: this applies to those not mentioned in HP lore he already knows mcgonagall, padfoot, prongs, wormtail, and skeeter the only ones I know are said to have one)