How can i tell if a sore on my arm is a pimple or a spider bite?

See the spider: Most of the the sores people get are furuncles or staph infections and common to think they are spider bites. The bite of a spider will usually start red and tender, but not raised. It can get infected in a few days. It also depends on the type of spider.

Dots: A spider bite usually has several dots in the red area. Also, the spider injects venom which causes a much larger area of redness and induration(roughness).