How can I tell if I was bitten by a spider this morning?

Depends. It is hard to tell. Redness, swelling and warmth at the site can be due to any insect bite. If there are any signs of an infection developing like oozing, pain at the sites, red streaking leading away from the sites, fevers associated withe the above symptoms, nausea, vomiting or if your tetnus is not up to date, or a black area at the center of the sites spreading outward (eschar), get seen!