How do you tell if you got bit by a spider or insect?

You really can't: Some insect and spider bites have a characteristic appearance, though generally you really can't tell what bit you unless you have the critter or the carcass in hand. The majority of insect bites do not need treatment unless from a poisonous species, or unless you have compromised health. If excessively large or swollen or infected, give you md a call. If you live in a lyme diseas area do see md.

See below: Generally, bug bites will cause a local reaction which can be small or large and includes: redness at the bite site sometimes with a center puncture wound, swelling, warmth and itch or no itch. If it becomes infected, the are will likely become painful and possibly ooze or have red streaks running away from the site toward the central part of the body. Sometimes other rashes can occur too.