Generally toward the end of the story, but there are a few caveats, because the universal theme of a story is often never explicitly revealed at all.
Authors want to leave their stories open to interpretation, and signposting a particular theme has the potential to backfire and make it less universal. For example, while writing, "And that's why everyone should brush their teeth before a job interview!" at the end of a story is certainly universal (see "everyone") and engages with themes of hygiene (let's say this was a story about the different ways that personal cleanliness--or lack thereof--can sabotage someone's interview), this sentence would feel trite and forced in most literature. If an author has a universal theme in mind, they will often employ it subtly.
For this reason, the universal themes of a story can usually be found tucked away in numerous places--within recurring imagery that evokes an idea, or even in the dialogue of a character who acts as a (quiet) mouthpiece for the author's message.
However, if an author does plan to reveal their universal theme explicitly, check the last 4/5ths of the book for clues, from the climax on. At this point in the story you have context from the journey the author has taken you on, and if the author is going to wrap things up nicely with a resounding message they hope will apply to all readers, this is the place for it--they've made it this far, and are looking for something lasting to take home with them and to resolve the story. Maybe it's a traditional hero narrative, and a protagonist up against a villain shouts across a battlefield to convince them that love is always better than hate. Maybe it's a smaller scale adventure, and someone back home reminds the protagonist that friendships last longer than arguments.
Of course, certain types of stories make their universal themes very explicit, like fables, which come with morals attached, though they can often be interpreted in a few different ways.
Happy reading!