Of course, there are many different versions and editions of this book in print, so it is very hard to give you the right page number.
In one edition of the book (sold on Amazon), the page you are looking for is page 47. In that book, Part 1 starts on page 1 and Part 2 starts on page 69. So the page you are looking for is about two thirds of the way through Part 1.
Another way to find it is to know that it is just a bit before the part where Guy gets sick. It happens as they are going to bed and then in the morning he's sick.
The page number will actually depend on the version of the book as indicated in the previous answer. You may have either a print version or a digital version. Whichever the case, the page that contains this information can be easily traced and falls towards the end of the first half of the book. It was after the fire fighting incident in which a woman declined to vacate her house opting instead to be torched alongside her books. This encounter definitely affects the entire fire fighting crew as they remain mum on their way back to their work station and Guy in particular is tremendously affected as indicated by his state of mind when he got home that night. He attempted to engage Mildred in a meaningful conversion but Mildred was distant. It was during this conversation that Guy asked Mildred if she had seen Clarisse only for Mildred to respond in a rather casual cold-blooded manner that she thought Clarisse was dead. She said, “…But she’s gone for good. I think she’s dead.” In the version whose link I have referenced below, this happens on page 22. The next day, Guy is unwell, does not report to work and even contemplates leaving his job. Captain Beatty pays Guy a visit to his home after he misses work.