Does Capote state earlier in In Cold Blood that Perry and Dick were executed?
To the best of my knowledge, Capote does not. Let us remember that Capote in this famous literary work was trying to write a kind of fiction non-fiction, if that does not seem to be a contradiction in terms. As a result, he merely reports the action of this famous murder case as it happened, and tries to do so in a detached narratorial voice. He does not jump ahead of himself and go to the end of the story, taking each event in order and attempting to present it in a manner that is as unbiased as possible.
At the same time, however, we need to remember that Capote published this book in the aftermath of the case and the execution of the two murderers, who had been dead for less than a year. It becomes clear therefore that it was read by a public who were already intimately familiar with the case because of the press attention it had received and already knew the ending. Therefore Capote did not need to reveal it to a readership that already knew how this text would turn out.
He does not expressly state it, but near the beginning of the novel he foreshadows their death by stating: "At the time not a soul in sleeping Holcomb heard them--four shotgun blasts that, all told, ended six human lives."
Since there are only 4 Clutters killed, he somewhat forshadows their executions, if one is familiar with the context of the novel (as it was written in a time period and set in a state where execution was the sanction for convicted murderers).