In hydropneumothorax there is fluid at the bottom and air on the top. There will be physical findings of pleural effusion at the bottom and signs of pneumothorax on the top. In addition you should try to elicit the following findings when you suspect hydropneumothorax.

Focused Exam

Upper Limit of Fluid can be recognized by:

Percussion
Scratch test
Egophony

The upper limit of dullness of pleural effusion is a straight line around chest. It was believed, one should elicit a higher level of dullness in axilla compared to anterior and posterior chest in pleural effusion. Now it is recognized that it is an optical illusion and our perceived dullness being high in the axilla is a prejudiced finding.

Shifting Dullness: When there is air and fluid, the fluid shifts easily to dependent position. The lung and air floats up. If dullness was elicited in lower axilla, by having patient assume a lateral decubitus position return of resonance can be demonstrated at the previous site of dullness. In prone position the previously dull pleural gutter will become resonant as the fluid shifts and lung floats up. Shifting dullness is demonstrable with ease when there is hydropneumothorax.
Radiologically it is easy to demonstrate shifting of fluid in pleural effusion also, but however it is not easy to demonstrate shifting dullness, as one can do with hydropneumothorax. This may be because of the capillary action, there is always fluid even in upper portion of chest.
Succussion Splash: Succussion splashes elicitable in hydropneumothorax. Percuss the upper limit of dullness. Place the stethoscope and hold it firmly just above the upper limit of dullness. Hug the patient and shake him moving along with him. You will hear a splash similar to the splash that can be heard over stomach containing air and fluid.