Mild gastro or a migraine. bad nausea with headache. how to tell if it is stomach virus or not?

Associated symptoms: I would look for other symptoms such as fever, muscle aches, rash, vomiting and diarrhea.

Tincture of time...: The best thing to do is wait a bit and see if other infectious symptoms develop such as a fever, muscle aches, etc, and see if the symptoms go away within 24 hours. Many GI viral infections are self-limiting and of short duration. Also, if you get frequent headaches with nausea then chances are it is not infections occurring repeatedly.