How do i tell the difference between an interstitial cystitis flare up and a bladder infection?

Ic: By having your doctor do a urine culture.

Urine culture: You will have a positive urine test and/or urine culture for bacteria with a bladder infection. Possible to get a bladder infection on top of ic, but urinalysis is typically nitrite negative & contain no bacteria negative with pure ic. Symptoms of a bladder infection will resolve with antibiotics and will not or may even increase with ic

IC vs UTI: Many times you can't tell the difference; and often a "flare" if ic can be precipitated by a uti. Check the appearance of your urine through a clear glass - if "cloudy" or foul-smelling it's likely to be a uti, but it could also be a UTI (early) even if grossly clear and without a odor. The best test would be a urinalysis and follow up with your urologist.