Summarize:

Check to see if the gums around your tooth have become red, swollen, and sensitive. You may notice a gum boil, a pimple-like formation on the gum near the infected tooth and up to the root. You may also be able to see white pus in the sore or around the tooth the pus is actually what causes the pain because it puts pressure on your tooth and gums. When the pus begins to drain, your pain will subside some. Bad breath or a foul taste in your mouth is another give away. This is directly related to the build-up of pus. If your tooth is severely infected, pus may begin to drain from the tooth or from a gum-boil into your mouth. It may happen suddenly, in a rupture of the abscess, and it will taste metallic or sour. It will also smell bad. Avoid swallowing the pus. An infected tooth can change in color from a yellow to a dark brown or grey color. This change is caused by the death of the pulp inside of your tooth, namely a "bruising" from dying blood cells. The dead pulp will emit toxic products, like anything that decays, that will reach up to the surface of your tooth through the porous passageways in your tooth. A tooth infection may well spread to adjoining areas of the body, especially if you have let it go untreated. For example, the infection can affect your jaw, sinuses, or the lymph glands under your jaw or in your neck. These latter may swell, feel tender, or be too painful to touch. While any tooth abscess is serious and needs treatment, please seek medical attention immediately if you have an infection that spreads. Because it is close to vital organs notably your brain such an infection can easily become life-threatening. Your body can react to an infection by raising your internal body temperature, causing you to run a fever. Normal body temperature can range from 97 to 99F (36.1 to 37.2C). A fever is usually a reading over 100.4F (38C). Along with fever, you may also develop chills, headache, or a feeling of nausea. You may feel weak and become dehydrated, so be sure to drink water. Seek medical attention if your fever continues to rise or does not respond to medication, or if you run a temperature of over 103F (39.4C) for several days.
Look for swelling or discharge of pus. Note any tooth discoloration. Check for swollen neck glands. Beware of fever.