Look carefully at the underside of the nail. The part of the nail that contains the quick is thick and grows straight away from the foot. Where the quick ends the nail will be thinner and hook slightly downward from the nail. This takes some practice so I recommend to start trimming at the tip and take only small portions of the nail off to avoid hitting the quick. Have some “quick stop” on hand in case you trim too close. Be aware that if you do not keep nails trimmed the quick does get longer and it becomes impossible to keep the nails short.