An applicator； an applicator set； a method for determining cleanliness of a surface using the applicator or the applicator set； and a system for determining cleanliness of a surface comprising the applicator or the applicator set. The applicator can include a marker composition and an applicator body capable of retaining the marker composition. The marker composition can include retroreflective particles and a dispersion medium, the blending ratio of the retroreflective particles being from 50 to 90 mass % on the basis of the total marker composition.
