How did Tony know that Ralph killed Pie-O-My.

The thing is, no-one knows that Ralph killed Pie-o-my. Its one of those things that was left open to interpretation on the viewers part.
I'd imagine Tony was under the impression that Ralphie killed the horse for a number of reasons; Ralphie hasn't exactly been the most controlled person in the family (Killing Tracy, turning down a drink with the boss, taking his time with the Jackie jr situation, disobeying Tony's orders to not draw attention to the garbage buisness...), he did just break the news to ralphie about him seeing Valentina and staright after that well-timed news....Raplhie fobbed off responsibility of Pie-O-My on Tony.
Don't forget, Tony never liked Ralphie. I think Tony took Ralph as the obvious scapegoat, and given Raplh's history......Tony acted on impulse (which Dr.Mefli wouldn't be too happy about )!!

Fistly, I think I'd change the name of the thread (via Edit) to something like "Pie-O-My." I'd hate for somebody who hasn't seen season four to read that!

Regarding your question: I think before Tony even stepped into Ralphie's house, he was dead certain that hew as going to make Ralphie the scapegoat, whether he'd done it or not. THe fact that Ralphie didn't deny the incident and even provoked Tony just gave the Boss justification in killing him, and thus making him feel better (for a short while) in taking his anger out on somebody.

The first time I saw it, I was so shocked that I tended to be on Ralphie's side, but after rewatching it, I'd have to say that Ralphie did light the fire.

Mick

I don't think Ralph lit the fire to kill Pie-O-My. I think this was just an example of Tony's screwed-up priorities and his letting his temper cloud his thinking.

I don't think Ralphie lit the fire. I think Ralph's outburst about it being just a horse was because Ralphie's son was in the HOSPITAL hooked up to wires and hoses like a vegetable. Ralphie was DESTROYED by it. He even confessed to a priest, and started that fund [or trust?]. Ralphie just didn't care about the horse and was mad that Tony seemed to be showing more of a care to the horse situation then his son's, hence the way he talked to Tony about it being just a horse and him eating beef all the time.

Unless they say otherwise, Ralphie did it. Plane and simple. It was him.

Negative. It was a freak occurrence and a terrible accident, that fire. Ralphie didn't do it. This was just a great example of Tony's mixed-up thinking causing him to make the wrong decision, business-wise.

They said it looked like an accident. But like I said until they say that someone else did it Ralph is suspect number 1 in my book

Yah, I think I agree with you Chris.

I agree with Don Vercetti. At the time, the only thing Ralph cared about was his son. I think that horse was the last thing on his mind.