To "check in on" someone means to visit, call, or write to them to find out how they're doing:

When someone new joins the team, I like to check in on them at least once a day.

Here are some situations in which you can "check in on" people:

• check in on your children who have been playing without an adult watching them
• check in on your employees to make sure that they're doing a good job
• check in on someone who's sick to see if they're getting better or worse