How can you tell the difference between food poisoning and a stomach virus?

I ask because my kid sister was recently on a band trip to Florida, and unfortunately she spent the entire ride from Florida to New Jersey throwing up. She ruined several articles of clothing and unfortunately not all of them were hers and she continued to feel really sick the following day, though the vomiting had tapered off a bit. She was okay by the third day, though really tired and sort of weak.

I assumed since she was on a trip eating what probably amounted to fast fast food for several days and several meals, plus fast food at rest stops (notoriously dodgy), she probably had a mild case of food poisoning.

She had no fever or anything.

I’m just curious here.