You have to specify what assay you mean. Keep in in mind that at neutral pH superoxide anion is protonated  and is present in the form HO2(.)

the mechanism for  non-enzymatic phenazine methosulfate-nicotinamide adenine dinucleotide (PMS/NADH) system which generates superoxide radicals,  reduce nitro blue tetrazolium (NBT) to a purple formazan. The active compounds in ur extracts inhibits the reduction of NBT by quenching the superoxide radicals..

check the attached file please

Protocols that are widely being used (that we also use) involve (i) generation of superoxide anion radicals (either by exposing riboflavin to light in presence of electron donors, or throught PMS-NADH system or  xanthine-xanthine oxidase system); and (ii) reaction of superoxide anion radicals with certaing molecules/biomolecules to yield a product that can be determined spectrophotometrically (production of nitrite in first case, formation of formazan in the second case and reduction of cytochrome c in third case); Higher the concentration of superoxide anion radicals more would the generation of nitrite, formazon or reduction od cytochrome C. Therefore, in presence of superoxide desmutase that uses superoxide anion radicals as substrate, one would expect decrease in the formation of nitrite, formazan or reduction of cytochrome C. Accordingly, higher the SOD activity lesser would be the formation of nitrite, formazan or reduction of cytochrome C.

Find the attached review paper.