Smaug doesn't recognize the scent of Hobbit, although he knows what dwarves and men smell like.
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Bilbo told Smaug a riddle to keep him distracted.
Smaug tells Bilbo not to trust dwarves:In return for the excellent meal [of the Company's ponies] I will give you one piece of advice for your good: don't have more to do with dwarves than you can help!Smaug-The Hobbit: "Inside Information"
Clue finder is one of the names that the invisible Bilbo Baggins uses when he boasts to Smaug the dragon.
His sense of smell. It is one of the main reasons why Gandalf chooses Bilbo as the 14th member of the Company (because while Smaug would be familiar with the smell of Dwarf, the smell of Hobbit "would be all but unknown to him").
The dwarves are Bilbo's friends for most of the Hobbit.