For your migraine example, here are things I would have assessed:
1. Does the patient have a hx of migraines? If s/he has never had one before, or has been well-controlled for a long time but then suddenly has a whopping migraine -- that's likely going to get worked up.
2. Any trauma?
3. Fever or other recent illness?
5. Any other neuro deficits or s/s?
In other words ... the differential diagnosis rules out other big & bad stuff before concluding that it's "just" a migraine. The old ER adage: "prove to me that you're not sick". (in some patients, this takes only 15 seconds
Having said all that -- migraines are unlikely to respond to p.o. meds. If there are no red flags in the differential (see above) nearly all are going to get 500mL or 1 liter IVF and your provider's cocktail of choice such as Benadryl/Reglan/Toradol. Hoping you don't work with a provider who has been sucked into giving narcotics for migraines -- the chance of a rebound headache is very high, and it's a very problematic road to go down.
Exceptions to the above would be: "I ran out of Maxalt and all I really want is an Rx for 5 days or so until I can get in to see my doctor."
The "migraine" whose s/s don't really match the migraine profile. It's not for me to say whether or not you have a headache ... but if you're noisily crunching away on Cheetos or potato chips ...
The frequent flyer migraine patient (and yes, I'll say it, there is a subset of these that include symptoms of "drama") who has not managed to follow up with neurology as previously directed.
A strong suggestion for you, OP -- listen to the docs' assessment of patients. You'll learn a great deal about differential diagnosis that way.