Genetics Based Question!!! Look in the Details please?

Scarlet is a recessive mutant in fruit flies (Drosophila melanogaster) with an orange eye color. You have a wild-type fly that you suspect may be heterozygous for scarlet. If the mutant allele is st and the wild-type allele is st+, what cross would you make to determine if your fly was st/st+ or st+/st+? Predict the results of this cross.