Accurate characterization of protein-protein interactions is essential for understanding and controlling protein crystallization and phase transitions in a wide range of multidisciplinary fields, from fundamental and materials research to various pharma and industrial applications. This study introduces an innovative use of Fluorescence Lifetime Imaging Microscopy (FLIM) with the non-conjugated molecular rotor Sulforhodamine-B (SRB) for the characterization and comprehension of protein interactions, offering a new, efficient method for screening protein phase transition conditions. As models of study, non-specific intermolecular interactions in Lysozyme and Bovine Serum Albumin (BSA) solutions were investigated under various precipitant agents (salts, polymers) and pH conditions. FLIM analyses reveal a strong correlation between SRB fluorescence lifetime and the transition from repulsive to attractive protein interaction state, with a pronounced increase of the lifetime around the solubility point, enabling robust qualitative solubility predictions. Complementary SAXS analysis confirms that SRB acts uniquely as a probe for protein interactions without influencing the system state. Further time-resolved FLIM analysis on SRB lifetime populations indicates that the sensitivity of the fluorescence lifetime to protein interactions is driven by variation in probe-protein interaction affinity upon screening of the protein surface charges. These findings and the proposed methodology are of high interest particularly for structural biology and applications reliant on protein crystallization, including the development of novel biomaterials.