In 1984 when Winston has "capitulated" nearly completely, how can you tell?Page 227.
First off, it states that he has in the narrative itself. It admits outright that "He had capitulated; that was agreed." So, there is that. Thus far, the narration in this book has been reliable, relaying exactly what Winston had thought and believed all throughout the book. So, we can trust that Winston had indeed capitulated because the narrator indicates it was so. Then, they were letting his body recover--if he hadn't given up and caved, they would still be starving and torturing him to get him to that point.
Beyond this, however, look at what he does in the next page. In his earlier life before the imprisonment, he had kept a journal. In that journal, he had written down his thoughts of rebellion and questioning. Now, the only thing that he can write is the Party's different pieces of propoganda: "Freedom is slavery. Two and two make five. God is power." His own independent thought have been almost eradicated by then; when he tries to express himself, he is only expressing the sentiments of the party.
Then, if you look at his reasoning in the pages that follow, it mentions over and over again that "he accepted everything" that the Party said; he even doubted the laws of nature and gravity, because O'Brien said they were not finite, but what the Party said they were. Then, beyond believing these things, he actually practiced shutting his mind down when it tried to logically argue against such impossible statements as two and two make five. He was actually self-training himself to be submissive to the Party's assertions.
So, his recovering body, his absence of independent thoughts as reflected in his writing, his belief in illogical statements of the Party, and his self-training to truly believe those statements are all indicators of his capitulation. I hope that helped; good luck!