From Longman Dictionary of Contemporary Englishcan see/spot/tell something a mile offcan see/spot/tell something a mile offinformal if you can see something a mile off, it is very easy to notice You can tell a mile off that he likes you. → mile
Examples from the Corpus
can see/spot/tell something a mile off• He's a hawkeye, and can spot one a mile off, like that faraway kestrel.• Our sportsdesk can spot from a mile off a person who can not tell an in-swinger from a bouncer.• But I think he's lovely, and you can tell a mile off that he likes you. 