Would 怎麼知道 or 怎麼知 be the right Mandarin equivalent to the Cantonese phrase 點知?
If not, what would be the right equivalent?
怎麼知道 would be correct in colloquial Mandarin; 怎知 is relatively literary-sounding in Mandarin and nowhere near as common 怎麼知道 is in Mandarin, nor as common as 點知 is in Cantonese.
For the record, in the early vernacular Mandarin Chinese of the 西遊記 Journey to the West (Ming dynasty), both 知道 and 知得 are used, but the latter has fallen out of currency. 知 was still attested in certain contexts though: "你知那三個？", which in modern Mandarin would be "你知道(是)哪三個(呢)？/ 你知不知道哪三個？"
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