"He eats a potato."
Translation:Αυτός τρώει μία πατάτα.
Is there a similar rule that identifies masculine nouns?
Yes: most nouns ending in -ος are masculine as well as pretty much all nouns ending in -ας, -ης, (-ες, -ους).
Neuter nouns generally end in -ι, -ο, -μα.
Feminine nouns generally end in -α, -η, (-ου).
Endings in parentheses are less common.
Nouns ending in -ος can be any of the three genders, but are most often masculine.