sharp-eyed 
adjective
Quick to notice things; observant: sharp-eyed readers may have already spotted this...
- The process begins with a trip to the bank, where a sharp-eyed analyst will review your finances.
- Using his feet he scuffed the sand to shape a desperate SOS message which was spotted by a sharp-eyed Coastwatch pilot.
- Family circulars sometimes suggested marital irregularities to sharp-eyed investigators.
Synonyms
observant, perceptive, sharp-sighted, eagle-eyed, with eyes like a hawk, hawk-eyed, keen-eyed, lynx-eyed, gimlet-eyed;
watchful, vigilant, alert, on the alert, on the lookout, on the qui vive, with one's eyes/open/peeled/skinned
informal beady-eyed
rare Argus-eyed