Is my spider bite poisonous? how can i tell?

Depends: If you have brown reccluse spider bite , it is very poisonous, you need to see a doctor righ away.But if you did not see the spider , then look for redness, swollen , pain that gets worse with tissues surounding the bite got swollen with spreading of redness.

Spider bite: Seek medical care asap if you think a spider bit you ; you have fever, severe pain (can be felt at bite site ; possibly chest, back or abdomen); abdominal cramping /rigidness or ulceration of skin at site. You may need a tetanus shot (if you don't know when your last shot was or it has been > 10 years). Also medical care if you develop signs of secondary infection.