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!