"This is a monkey."
Translation:Αυτός είναι ένας πίθηκος.
In general: by looking at the shape of the word, i.e. the ending.
Words in -ού such as μαϊμού "monkey" (from maymun) are feminine, like the native αλεπού "fox".
Words in -ες such as καφές "coffee" (from kahve) are masculine.
Words in -ι such as τζάμι "glass" (from cam) are neuter, like the native παιδί "child".
Most older loanwords from Turkish get put into the neuter -ι category if they ended on a consonant in order to make then inflectable in Greek.
Newer loanwords are often borrowed without an ending and are then undeclinable and (usually?) neuter, e.g. το ασανσέρ ("lift, elevator", from French ascenseur).
Are πίθηκος, μαϊμού synonyms? SokratisMp says they are distinct. I ask because Mandeson (1961) lists the verbs μαιμουδίζω και πιθηκίζω as meaning "to ape, mimic, imitate." I know google translate is unreliable, but it gives both Gk nouns for monkey and ape, but lists μαϊμού first for monkey and πίθηκος first for ape, which coheres with SokratisMp's point.