Would 怎麼知道 or 怎麼知 be the right Mandarin equivalent to the Cantonese phrase 點知?
點 in Cantonese can mean 'how'. It is equivalent to 怎 / 怎麼 /如何 in Mandarin

(點)辦 = (怎)辦

噉(點)得 = 這(怎麼)行 

(點)算好 = (如何)是好

(點)知 = (怎)知