How do you know if the antibiotic will work if the strep might be antibiotic-resistant?

Depends: Strep a has been universally sensitive to the penicillin drugs & treatment within 9 days of first fever stops progression to the antibody production that can cause rheumatic fever. Sometimes total eradication from the throat is difficult but not considered a true failure. Erythromycin is one med that has seen true strep resistance in some parts of the world.