Myth: Doctors can always tell what spider bit you from the bite alone.
Fact: If they could, there wouldn't be hundreds of cases annually mis-diagnosed as "brown recluse bite" in regions where the spiders being blamed don't exist.
There are so many possible causes of bite-like sores that even an expert on venoms and arthropod bites can't always be sure of the cause in a particular case. Medical students don't even get one full day's training in spider-related topics. Researchers in Colorado and Michigan have developed techniques to diagnose bites by biochemical lab work, but currently few labs offer this service, which only works if done within a week after the bite occurs. I myself am often asked to advise people with "mystery bites" but I can't; my specialty is spiders, not bitten humans.
The moral of this is, if you see a spider bite you (an event of once-in-a-lifetime rarity), or any other biting or stinging creature for that matter, save the specimen for an arachnologist (and only an arachnologist!) to examine. Don't give it to someone who knows nothing about spiders.
Information on this web site is not a substitute for professional medical advice, and should not be used to diagnose or treat a medical or health condition. You should consult a physician as to any symptoms that may require diagnosis or treatment. Genuine spider bites can sometimes require medical attention, but beyond that, several medical conditions commonly mistaken for spider bite can be even more serious. If you have what appears to be a serious spider bite, please contact your health care provider or local emergency services. If you have the actual spider that bit someone, always save it for identification by a professional arachnologist.