How do I know if I have an infection of some sort?

Infected? That depends on the infection. Some infections have no symptoms. If you are concerned about a specific exposure, see your doc and get tested. For many infections there are specific symptoms: fever, sweats, chills, weight loss, cough, chest pain, vomiting, diarrhea, urinary discomfort/back pain, discharge, skin lesions with pus. For these you need to see a doc for treatment.