Difference between alveolars and alveolo-palatals in Cantonese
In essence, palatalisation of the alveolar fricative and affricate consonants occurs before front rounded vowels /y/, /œ/ and /ɵ/, with the effect strongest with /y/. It is weakly palatalised in front of /œ/ and /ɵ/, to much less of an extent than Mandarin Pinyin j-, q-, x-; however, there is some variation among speakers. See Matthews &amp; Yip (2014) Cantonese: A Comprehensive Grammar, page 474, as well as this article from Zee (1997).

It used to occur with the front unrounded vowel /i/ as well: see the entry of palatalization in Bauer &amp; Benedict (1997), so I'd imagine some of the older generation would preserve this form of allophony. I'm not sure if there's anything in the literature as to variation in this part of diachronic Cantonese phonology!