How did smaug know that bilbo was traveling with the dwarves?

Smaug could smell the dwarves.

"Don't talk to me!" said Smaug. "I know the smell (and taste) of dwarf-no one better. Don't tell me that I can eat a dwarf-ridden pony and not know it! You'll come to a bad end, if you go with such friends. Thief Barrel-rider. I don't mind if you go back and tell them so from me."