While I am no expert, and have never owned either....
I read somewhere that the nuchal plate has a small lip or curve on it that is only present on Aldabras. This curve on the nuchal plate is the only way to distinguish between the two. The article also mentioned that even the head shapes can at times be difficult to tell apart. (I read this somewhere online, I'm not sure where though.)
I do not know if this "curve" is present just after hatching, or if it something that emerges later with aging.
You might want to contact the Captive Born Wildlife Foundation. The owner has and breeds both tortoises and might be able to give you a definite answer.
-Kory
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I've seen a few photos recently of Galap hatchlings that looked exactly like aldabs. Does anyone know if Galaps sometimes have a nuchal plate like an aldab, and if so how else do you distinguish hatchlings ? In older animals the head proportions are clearly different but not so much in juveniles.