How do i know if a pimple is infected?

Look for these clues: Infection usually shows up with redness, warmth, swelling, and pain. But there may also be pus that drains - and will actually help clear the infection. Be very, very careful about squeezing infected pimples, however, because an infection can easily spread in this manner. Rather, warm compresses held over the area may help an infected pimple drain naturally. See your doctor if you have any doubt.