Check In

AmE check in
to leave your bags at an official place so they can be put on a plane 
or a train, or to take someone's bags in order to do this
　Any luggage over five kilos must be checked in.
AmE to call someone to tell them that you are safe or where you are
　He just called to check in and tell them how he was doing.
check on / [check on sb/sth] phr v
to make sure that someone or something is safe, is in a satisfactory
 state, or is doing what they should be doing
　Honey, can you go upstairs and check on the kids?
　My neighbour comes in once a week to check on things and feed the fish.
to try to find out if something is true or correct
　He wanted to check on the girl's story.