Most people prefer to know immediately if they have spinach between their teeth, a visible bra strap, an undone zipper, a stain on their clothing, smudged makeup, or any other embarrassing issue, rather than discovering it an hour later.
Choose an appropriate moment to discreetly and privately alert them without drawing attention from others. Be respectful and tactful in your approach. With a touch of finesse, you might say, “Pardon me, but your slip is showing,” so they can quickly and privately fix the issue. They’ll appreciate your help in preserving their dignity and self-esteem.