How does Bilbo know that the three people are trolls?

Answer:

As it says in the book, they were obviously trolls, so obvious that

Even Bilbo, in spite of his sheltered life, could see that: from the great heavy faces of them, and their size, and the shape of their legs, not to mention their language, which was not drawing-room fashion at all, at all.
Contributor: Aggie80
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