be on the lookout for
• be alert to (danger or trouble).
• "he told them to be on the lookout for dangerous gas"
Similar:
keep watch,
keep/be on one's guard,
beware,
keep an eye out,
keep a vigil,
be alert,
be observant,
be attentive,
be on the qui vive,
keep one's eyes peeled/skinned,
