Been taking keflex 500mg 4 times daily for sinus infection. Have no symptoms other than low grade fever. How or when will I know if its going to work?

See below. Sinusitis is characterized by nasal congestion, purulent nasal discharge, headache and facial pain over the sinuses in the frontal and maxillary areas. Low grade fever alone would not indicate sinusitis. Sinusitis is usually self limited and does not require antibiotics unless severe or prolonged. Keflex is not the best drug for sinusitis. Ceftin, (cefuroxime) vantin or Augmentin would be better choices.
Depends. Keflex is a cephalosporin type antibiotic, which is rather broad spectrum. It can be bacteriostatic (knocks them down) or bacteriocidal (kills them-bacteria). I do not know how long you have been taking it, but a full course of 7 days needs to be taken. If not better by then, a culture and sensitivity test should be done to find the appropriate drug for a full cure.