"onks" can stand for "onko" in spoken Finnish. The sentence decides its meaning.
Spoken form of 'onko': the -ko question ending becomes -ks
onks
These are related spoken alternatives. Which one you hear varies by speaker and situation. Check each example rather than treating them as interchangeable in every sentence.
Onks sul nälkä? - Are you hungry?
onks here: is (there)? - spoken form of 'onko'
Written Finnish: Onko sinulla nälkä?
Onks se kaukana? - Is it far?
onks here: is? - spoken form of 'onko'
Written Finnish: Onko se kaukana?
Onks sauna jo lämmin? - Is the sauna warm yet?
onks here: is? - spoken form of 'onko'
Written Finnish: Onko sauna jo lämmin?
Moi, onks tääl tilaa? - Hi, is there space here?
onks here: is - spoken 'onko': the -ko question ending becomes -ks
Written Finnish: Moi, onko täällä tilaa?
Onks tää bussi? - Is this the bus?
onks here: is - spoken 'onko': the -ko question ending becomes -ks
Written Finnish: Onko tämä bussi?
Onks toi juna? - Is that the train?
onks here: is - spoken 'onko': -ko becomes -ks
Written Finnish: Onko tuo juna?
Spoken and written Finnish are different registers. The examples show how "onks" is used here and pair it with the written sentence. Learn the meaning in context rather than replacing every occurrence of a word.