hard to fool

• shrewdPractically clever, perceptive
• sharpFine-edged, or astute
• savvyPractical shrewdness
• astuteMentally sharp, discerning
• skepticalInclined to question claims

suspicious · wary · perceptive · cunning · discerning · nobody's fool · streetwise · canny · watchful · alert · quick · clever · wise · guarded · doubtful · penetrating · clear-eyed · hawk-eyed