For a long time, your kid can't actually tell you when something's wrong. Or what's wrong. You have to get smart about figuring it out. Looking for the signs. Learning the kid's language of nonverbal clues.
But at some point, the kid will be able to talk about what's going on. From then on, you want to be sure you keep the lines of communication open. Make sure the kid feels she can tell you something without fear of embarrassment, or without worrying that you'll get angry or annoyed or whatever for messing up your grand plans. She needs that comfortable space where she can express herself. Share what's bothering her. Her concerns, her fears, her--
Look. Here's the deal. You just really, really want your kid to tell you her stomach's not feeling well before you feed her five tamago, a tobiko roll and a cucumber roll, half a bowl of edamame, and a snack-size Oreo McFlurry, not after she's puked herself awake all over the guest bedroom at your mom's house.