yaumingfai's suggestion 「我講咗架喇！」for " I told you so" is pretty close to a word by word translation，other suggestions are good too, of course there are many many way to rub it in to people,
(1) When someone didn't take your warning and made mistake just like you predicted , you can rub it in with:
「係咪呢？」 (see?)
「話/講你又唔信」 ( I told you, but you didn't believe.)
「話/講你又唔聽」 ( I told you, but you didn't listen.)
「聽我講咪冇事囉？」( listened to me, everything would have been fine now.)
(2) when your prediction come true, and you want to brag about it you can say:
「我冇講錯喇掛？」( I didn't say thing that's incorrect, did I ?)
「仲唔俾我講中？」( finally, it is as I predicted .)
(1) When someone didn't take your warning and made mistake just like you predicted , you can rub it in with:
「係咪呢？」 (see?)
「話/講你又唔信」 ( I told you, but you didn't believe.)
「話/講你又唔聽」 ( I told you, but you didn't listen.)
「聽我講咪冇事囉？」( listened to me, everything would have been fine now.)
(2) when your prediction come true, and you want to brag about it you can say:
「我冇講錯喇掛？」( I didn't say thing that's incorrect, did I ?)
「仲唔俾我講中？」( finally, it is as I predicted .)