A non-invasive, image-based analytic method utilizing scattering-type scanning near-field optical microscopy (s-SNOM) is suggested to evaluate the phase separation behavior of lamella-forming polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) block copolymer films. Taking advantage of the penetrability of the tip-enhanced IR signal into the films, the spatio-spectral maps of each component are constructed. Subsequently, the effect of a sole and combinatorial applications of the self-assembly procedures, such as solvent vapor annealing (SVA) and/or thermal annealing (TA), on the spatial distribution of PS or PMMA components is quantitatively assessed in terms of the areal portions of the PS domain, PMMA domain, and the mixed zone that is adjacent to the domain border. Additionally, by statistically comparing the local concentration profiles, the chemical contrast between the domains turns out to be dependent upon the annealing procedures (namely, SVA and SVA + TA). This technique can pave the way to an uncomplicated but precise investigation of the polymer nanostructure-based thin film devices whose performances are critically governed by the spatial arrangement of the chemical elements.