Ever heard the saying "all patients lie"? Well, I'm not completely condoning that statement, but there can be some pretty big discrepancies between the patient's version of the truth and what's really going on. If there are others on-scene who are not completely hysterical, ask them if they know any of the information. If you get info, cross-reference that with what the patient says. But also trust yourself and your perception. If you see pill bottles, but your patient insists they have no prior medical hx, then check the meds, see what's there. Another sign, though albeit a bad one, is when the pt. has a bag filled with meds. Sure sign that there's a lot more going on than what you might learn from the pt. Also, use your senses. We had a pt. who said that she couldn't feel her arm. We quickly discovered the source of the disability, not with sight or touch, but with smell. Her left brest and side were completely necrotic; the necrosis ravaged her arm, rendering the musculature useless.
Basically, be your own best judge.