I wondered this recently. Here's what the doc told me (memory is poor, so I hope someone in the know will correct me): Look for a low-grade fever, and pain during physical activity. If she complains more while running, for example-- it might be because the inflamed appendix is being jostled. Try a rebound test: press on her belly, then let up quickly. See if that hurts. Pain will often be on the LEFT side (even though the appendix is usually on the right). When I did this to my four-year-old, she giggled and pointed right at her belly button when I asked her if/where it hurt. DO worry if a severe bout of pain, high fever and nausea is followed by sudden relief. A burst appendix will often feel better for a bit-- before you die of sepsis.
(Now that I read that, it's not reassuring at all. Could be anything, or nothing.

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My kid just had a bug, which makes sense, because her dad had a horrible one that same week. He had severe pain, general malaise, and diarrhea; she just had pain. She was very perky between belly cramps. It cleared up in 2-3 days.