We combine scanning force microscopy experiments with ex-situ swelling in different solvent vapors to investigate the microdomain structure of a thin ABC triblock copolymer film (poly(styrene-b-2-vinylpyridine-b-tert-butyl methacrylate)). We demonstrate that short treatment in a selective vapor and subsequent drying lead to characteristic changes in the surface morphology. The use of different solvents then allows to unambiguously identify the different phases present at the surface. This approach is established studying polymer blend and diblock copolymer copolymer thin films of known known morphology. It is then applied to the a priori unknown morphology of the BBC triblock copolymer thin film. The results indicate a laterally microphase-separated polymer surface in agreement with recent theoretical considerations, The conclusions are corroborated by XPS measurements monitoring the average surface composition of the copolymer films.