How does a doctor determine whether a child is experiencing migranes?

I couldn't see your whole question.  A doctor figures out if a child has migraines by taking her story into account, the family history and if the headaches are frequent or increasing, or affect school or social lives or have alarming characteristics --doing tests such as a CT or MRI to rule out more serious illnesses.  Diagnosing migraines is mostly clinically  -- that is listening to and examining the patient.

I just saw the rest of your question.  I would agree that Topamax for an 8 year old that you have never known to have a migraine should be questioned.  Topamax would not be a first line pick for a child with migraines.  Normally preventatives are prescribed when people have 4 or more migraines per month  (of course, there are other considerations like length of headache, specific headache symptoms etc). Topamax is quite safe, but has side effects including increased risk of kidney stones, Decreased sweating -- increasing the risk of heat related illnesses.  Most people on Topamax will have tingling sensations.  Topamax can dull your thinking, making it difficult for people to find words when speaking, I would be concerned that it could affect school performance.  Topamax carries the knickname Dopamax in the medical community  - for good reason  -- A sharp neurologist can pick up if a patient is on Topamax just by carefully observing them.  Topamax also is being studied as a weight loss drug in adults.  Pediatricians and Neurologists would normally choose medicines that have been around a long time because they know more about them. Medicines such as Periactin or a Beta Blocker would be considered first line for kids they have been used safely for many years in kids.  Medicine knows more about them. 

Iammelanie,

It's my understanding that Topamax was in clinical trials for weight loss a couple of years ago. The trials were started twice, but discontinued both times.

Teri