If you check up on someone, you obtain information about them, usually secretly. I'm sure he knew I was checking up on him
If you check up on something, you find out information about it. It is certainly worth checking up on your benefit entitlements The Government employs tax inspectors to check up and make sure people pay all their tax. see also check-up
check-up check-ups in AM usually use, and in BRIT sometimes use, checkup A check-up is a medical examination by your doctor or dentist to make sure that there is nothing wrong with your health. The disease was detected during a routine check-up
[ 'chek ] (noun.) 15th century. Middle English chek, from Middle French eschec, from Arabic shAh, from Persian, literally, king; akin to Greek ktasthai to acquire, Sanskrit ksatra dominion.