窺いしる[ukagaishiru] to perceive
窺う[ukagau] to peep (through)
窺知[kichi] perception

窺い探る[ukagaisaguru] to spy out
窺く[nozoku] to peek (though a keyhole, gap, etc.)
隙を窺う[sukiwoukagau] to watch for an unguarded moment

覗う[ukagau] to peep (through)
覗き[nozoki] peeping
覗き見[nozokimi] peeking
覗きこむ[nozokikomu] to look into
覗く[nozoku] to peek (though a keyhole, gap, etc.)

覗魔[nozokima] peeping tom
管の穴から天を覗く[kudanoanakaratenwonozoku] Have a narrow view of things
覗眼鏡[nozokimegane] peep show
覗機関[nozokikarakuri] peep show
差覗く[sashinozoku] to peek