Depends on Severity
This would depend on the severity of the disease state in the individual, but by and large, it's enough of a difference from "normal" sweat to make one take notice.
Clinically, the sweat chloride
level is measured twice to make the diagnosis of cystic fibrosis
(cf). The cutoff level to make the diagnosis of CF is 60 milliequivalents per liter of sweat chloride.
Answer assisted by David Escobar, Medical Student