be (one's) calling card

1. To be something, typically a special talent or skill, that sets one apart from others. His ability to create such lifelike portraits really is his calling card in the art world. Of course Shelly wrote that scene—witty dialogue like that is her calling card. Infrared photography is really your calling card these days, huh? I could pick out your prints immediately.

2. To be something that one leaves behind to signify that they have been in this particular location. A: "All we found at the scene of the crime was a single white rose." B: "That's definitely the same killer's calling card." We have to leave something so they know the same people broke into their house again—it'll really freak them out! Hmm, what can be our calling card? Smashed mirrors seem to be this thief's calling card.