How did Mama and Papa bear know someone was sitting in their chairs?

Question

Goldilocks sits in Mama and Papa bears' chairs, but does not break them:

After she'd eaten the three bears' breakfasts she decided she was feeling a little tired. So, she walked into the living room where she saw three chairs. Goldilocks sat in the first chair to rest her feet.

"This chair is too big!" she exclaimed.

So she sat in the second chair.

"This chair is too big, too!" she whined.

Later, when the bears come home, they somehow discover someone had been sitting in the chairs:

"Someone's been sitting in my chair," growled the Papa bear.

"Someone's been sitting in my chair," said the Mama bear.

What did Mama and Papa bear see that led them to believe someone had been sitting in their chairs?

Answer 1

According to the 1839 edition of Robert Southey's The Story of the Three Bears it was because their chairs' cushions were disturbed:

"Somebody in my chair has sat!"
With voice so gruff and great
The Huge bear said, when he saw that
His cushion was not straight.

"Somebody in my chair has been!"
The middle Bear exclaim'd;
Seeing the cushion dinted in
By what may not be named.

The wording was slightly different in the first edition (of 1837). The history of this story (and its publication history) is interesting and complex: see the Wikipedia article on it. In particular, the identity of the home invader has changed.

Answer 2

The chairs were disorganized and put in an unorderly fashion, suggesting that they had recently moved, along with the seat cushions (as stated in the other answer).