When it is clearly Asymmetric

Look at the point where the two hemispheres of the mushroom body meet. If there is an apparent tilt towards the left/right (from your viewpoint in the stack) due to one hemisphere of the Medial Lobe being larger and more dorsal to the other, mark them as egocentric right/left accordingly (since we are looking from the front of the animal, anterior to posterior, the egocentric leaning is the opposite of what we see).

When it is hard to identify the Asymmetry

Sometimes, the leaning/dominance of the hemisphere is a little unclear; at that point, you can adjust the brightness using the Fiji brightness/contrast tool to see if the tilt is more evident. Focus on the area slightly posterior to the first point of contact when going from anterior to posterior. Two sub-lobe-like structures can be seen at this point. Note that the one that is higher up at the dorsal edge of the mushroom body's medial lobe, right under the central body, is likely the dominant lobe. Another way to look at it is the boundary of the sub-lobes on the less dominant side meets the other hemisphere on the L/R boundary. If the left side is more prominent and higher up from your viewpoint, the effective leaning is right-leaning, and therefore, the brain is egocentric left-leaning and vice versa.