Be vigilant, be on the lookout or be careful
(Verb)
Synonyms:
  1. watch
  2. take-care
  3. heed
  4. be cautious
  5. proceed carefully
  6. mind
  7. go on tiptoe
  8. take precautions
  9. be on one's guard
  10. make sure of
  11. be doubly sure
  12. look alive
  13. keep-an-eye-peeled
  14. handle-with-kid-gloves
  15. look out