a calling card
1. A small card containing one's contact information; a business card that may also be used for social purposes. I'm old school, so I like to hand out a calling card when I'm saying goodbye to a new acquaintance.
2. An item left behind by someone to signify that they had been present in a particular location. It was evident that the string of murders were committed by the same man, as he left his calling card, a single red rose, at the scene of each crime.
3. A special skill or talent that sets an individual apart from others. Primarily heard in US. His ability to paint such lifelike portraits really is his calling card in the art world.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
a calling cardmainly AMERICAN
1. If you describe what someone possesses or has achieved as a calling card, you mean that it gives them an advantage. An Olympic gold medal is undoubtedly a pretty impressive calling card.
2. If you describe something as a calling card, you mean it proves that someone has been in a place. The hand continues to leave behind its calling card on everything it touches in the form of fingerprints. Note: In American English, a calling card is a small card printed with your name and other personal information, which you give to people when you visit or meet them.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012