Spider bite or pimple, how can i tell?

Can't always: Spider bites can have variable appearances. It might cause a pink or red bump with no symptoms. Or it could itch or be painful. Venomous spiders may cause significant pain radiating far from the site, cramping, sweating, necrosis of tissue or worse if there is an allergic reaction. A pimple will usually arise in particular areas of the body ; would not be accompanied by severe pain or necrosis.