The formation and characterization of solid solutions with guest (or impurity) molecules are difficult to realize in crystalline organic semiconductors. Here, we demonstrate that an operando field-induced electron spin resonance (FIESR) experiment allows the site-selective analysis of accumulated charge carriers in polycrystalline organic field-effect transistors (FETs), which comprise molecular solid solutions as channel semiconductor layers. We utilize vacuum-deposited dinaphtho[2,3-b:2',3'-f ]thieno[3,2-b]thiophene (DNTT) thin films with various amounts of dibenzotetrathiafulvalene (DBTTF) as guest molecules. Our measurements reveal that the field-induced carriers are first trapped at guest DBTTF sites at low gate voltages and then begin to accumulate at host DNTT sites at higher gate voltages. Furthermore, we perform dispersion-corrected density-functional theory calculations to investigate the stability of the solid-solution crystals. It is shown that the DBTTF molecules can be stably incorporated by aligning molecular long axes within the crystal lattices of DNTT, the results of which agree well with the angle-dependent FIESR measurements. We demonstrate that the mobility, threshold voltage, and subthreshold swing are controlled by the concentration of guest molecules in solid-solution organic FETs.