In chapter 7 of "To Kill a Mockingbird", what hints are given that Boo sewed up Jem's pants?
First of all, the pants are folded nicely across the fence, something that only a friendly individual who knew the habits of the children would have done. Jem is creeped out a bit by this, saying they were sitting there "like they were expectin' me". We know already that Boo watches the children playing (the flick of the curtain as Jem slapped the house, the laughter as Scout flew from the tire, and the fact that he knew they passed by the tree every day and so left gifts for them). So, he probably guessed that Jem would have to go get the pants so as to not get in trouble.
On top of this, Jem states of the sewing job that "they'd been sewed up. Not like a lady sewed 'em, like somethin' I'd try to do." So they were sewed up by someone not used to sewing, and the implication is that a male, unpracticed with the fine art of sewing, did it. The logical conclusion of all of this is that Boo got the pants and fixed them up.