In The Wise Man's Fear:The Kingkiller Chronicle: Day Two chapter 130, Wine and Water Kvothe comes in contact with a band of false troupers, who eventually reveal they've done some bad things. At some point in the encounter he poisons them. But based on the chapter it seems he does this before they reveal their crimes. Did Kvothe simply use the Zen analog Lethani to decide to poison them? Did he see evidence of their crimes prior to meeting them? Was there something in the way they did the water and wine ceremony that tipped him off that they were fasle Edema Ruh? What drove his decision?
When Alleg offers Kvothe ale, he points out that they nicked it in Levenshir. At that moment, Kvothe knows that these people aren't Ruh, because Ruh wouldn't steal.
The best case scenario is that he's just wandered into a camp of bandits who stole Edema wagons, and I suspect that he initially poisoned them to make an escape (he later says that the poison won't kill, it'll just make someone sick for a while)...but his plans changed when he became aware of the women they'd kidnapped.
As Liesmith points out in their answer, Kvothe decided to poison the false troupers after learning that they stole the ale from Levinshir; however, all evidence also points to him deciding to kill them at this point as well.
Describing his motivation to Krin (one of the kidnapped girls), he says :
"For pretending to be Ruh? No. ... For killing a Ruh troupe and stealing their wagons? Yes. For what they did to you? Yes." (page 868)
After promising to travel with the false troupers "until no one objects to [his] leaving" (858) (before he learns of the girls), Kvothe says, "I swear on my mother's milk, none of you will ever make a better deal than the one you made with me tonight." Similarly to his comment on the poisoned stew ("Anyone who does not enjoy this fine stew is hardly one of the Ruh in my opinion." (857)) these comments appear to compliment the bandits, but actually ironically express his plan to kill the bandits.
The only thing I can find is on the second page of the chapter.
"A thick-bodied man wearing a sword stomped out of the trees. 'I'll be damned if he came past me, Alleg. He's probably from...' 'He's from our family,' Alleg interjected smoothly."
This must have been enough to tip Kvothe off that something was off about this troupe. I can't find anything else.
He offered him and after the water instead of the wine, that plus the fact that he said he stole it were the first two dead giveaways, then the inhospitable granny? That is not ruh