Your doctor will ask about your symptoms and how you were bitten. He’ll want to know if you saw a spider bite you, and if you did, what the spider looked like. That’s really the only way he can know for sure that it was a spider that bit you.
If you have more than one bite on different parts of your body, or if several people in your house were also bitten, a spider is probably not to blame. In this case, your doctor will examine you to rule out other causes, like an infection.