Same word as observant:

advertent, alive, attentive, bright, clear-sighted, comprehending, considering, contemplating, correct, deducing, detecting, discerning, discovering, discriminating, eager, eagle-eyed, heedful, intelligent, intentive, interested, keen, mindful, not missing a trick, obedient, observative, on one's toes, on the ball, penetrating, perceptive, questioning, quick, regardful, searching, sensitive, sharp, sharp-eyed, surveying, understanding, vigilant, wide-awake