This is an example of significant fluid in the Pelvis that actually should not be mistaken for the Bladder. The reason you could tell it's not Bladder is that tracks into areas around the Bal although on a single transverse it may actually look like the Bladder. So this is a potential pitfall.
In the left upper quadrant we have to be aware that the stomach is present there. If there's any fluid in the stomach it may appear as a fluid filled structure, but again this is an encapsulated structure, in this case we can see some hyperechoic air bubbles in there, but the fluid does not track between the Spleen and the Kidney.
When we're interrogating the chest we want to be careful about differentiating Pericardial from Pleural fluid. This image actually shows both. In this case there's a small pericardial effusion which tracks between the posture board of the heart and the descending Aorta. There's also a left Pleural effusion that is lateral to the descending Aorta and should not be mistaken for a pericardial effusion.