1) Blew the whistle 2) Cautioned 3) Forewarned 4) Gave a heads-up 5) Gave a heads-up to 6) Gave a warning 7) Gave a warning to 8) Gave fair warning 9) Gave warning 10) Given a tip 11) Given a warning 12) Given advance notice 13) Let know 14) Made aware 15) Made vigilant 16) Now in the know 17) On guard 1) Cautioned