Dullness on percussion and decreased breath sounds indicate the presence of fluid in the pleural space, which impedes sound transmission through the lung. Fluid accumulation causes the normally resonant lung area to become dull and muffles breath sounds because the fluid blocks both the air and sound waves. This is a classic finding in pleural effusion. Other options like hyperresonance or bronchial breath sounds point to different pathologies such as pneumothorax or consolidation. Therefore, dullness and decreased breath sounds are the most reliable signs to suggest pleural effusion during examination.

Reference: Bates' Guide to Physical Examination and History Taking, Lynn Bickley, 13th Edition.