I stepped on a cactus 2 weeks ago and it is now blistering. i think there is still some needles in my heal because it hurts to walk on it. how do i know if it's infected?

Watch where you step: Infection usually shows itself with classic signs. These include redness, swelling, pain and increased warmth in the area. If you are unable to remove the needles, then I suggest having someone else, who is better equipped for the task, do it. Call your family doctor for directions as to who to see. Foot specialists often have the necessary equipment and will more than willing to help.